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### Models of the Origin of Avian Flight

How theropod dinosaurs evolved the ability to fly remains a subject of intense scientific debate. Three models have been proposed to explain the evolutionary pathway, behaviors, and aerodynamic forces that led to powered avian flight.

Model 1 (Arboreal Model)
This model proposes that the ancestors of birds were tree-dwelling (arboreal) organisms. These proto-birds jumped between branches and trees. Over time, selective pressures favored morphological adaptations that increased surface area, allowing them to parachute, then glide, and eventually achieve powered flight. In this model, gravity served as the initial energy source, reducing the metabolic cost of early flight stages. Flapping flight evolved as a means to extend gliding distance and control landing.

Model 2 (Cursorial Model)
This model proposes that avian flight evolved in bipedal, ground-dwelling (cursorial) theropods. These active predators ran along the ground to capture prey or escape danger. They utilized proto-wings to assist in balance, increase running speed, and control leaping maneuvers. Powered flight evolved directly from horizontal running as the animals generated sufficient thrust to achieve takeoff velocity. Gliding was not a precursor; flapping behavior arose to increase thrust and lift.

Model 3 (Wing-Assisted Incline Running Model)
This model proposes that the precursor to flight was wing-assisted incline running (WAIR). Proto-birds used their forelimbs not to glide or generate takeoff lift, but to run up steep or vertical surfaces (such as tree trunks or cliffs) to escape predators. By flapping their proto-wings, they generated aerodynamic downforce (similar to the spoiler on a race car), which pressed their feet against the incline, dramatically improving traction. As the stroke angle shifted, this downforce behavior transitioned into powered flight.

Match each aerodynamic mechanism or energy source on the left to the corresponding model description on the right.

### Enceladus Plume Sources

Scientists are investigating the source of the plumes of gas and ice grains erupting from the south polar region of Saturn's moon, Enceladus. Two models have been proposed:

Model 1 (Subsurface Ocean Model)
The plumes originate from a global liquid water ocean situated between Enceladus's icy outer shell and its active, rocky silicate core. Tidal forces flex the core, causing hydrothermal activity (temperatures $> 90^\circ\text{C}$). This water-rock interaction dissolves silica ($SiO_2$) and produces molecular hydrogen ($H_2$) via chemical reactions. Upwelling currents transport the water, dissolved silica, and dissolved gases to the surface, where they erupt through fractures in the ice shell as gas and ice grains containing silica nanoparticles.

Model 2 (Clathrate Hydrate Model)
The plumes originate entirely within the shallow, icy outer shell. The shell contains clathrate hydrates—structures of water ice that cage gas molecules (primarily $CH_4$ and $CO_2$) under high pressure. Tidal forces cause friction along fractures in the ice, heating the surrounding ice to temperatures well below $0^\circ\text{C}$. This localized heating causes the clathrates to decompose, releasing the trapped gases, which then escape into space. Because this process occurs entirely within the cold ice shell, there is no high-temperature water-rock interaction at the core to produce silica nanoparticles or free $H_2$.

A spacecraft analyzes the composition of the ice grains ejected from the Enceladus plumes. The analysis detects significant amounts of silica nanoparticles ($SiO_2$) and molecular hydrogen ($H_2$) gas.

Based on the models, does the detection of silica nanoparticles and molecular hydrogen in the plume ice grains support Model 1, Model 2, or both?

### Origin of Earth's Water

How Earth acquired its water is a subject of debate among planetary scientists. Two models describe different origins:

Model 1 (Asteroid Delivery)
Earth initially formed dry because the heat of the early Sun drove volatile compounds outward. Later, water-rich carbonaceous chondrite asteroids from the outer solar system collided with the cooling Earth, depositing water. The Deuterium-to-Hydrogen ($D/H$) ratio of these asteroids matches the ratio found in Earth's current oceans ($1.5 \times 10^{-4}$).

Model 2 (Nebular Ingestion)
Earth acquired water during its formation. The proto-Earth was surrounded by hydrogen-rich solar nebula gas. This primordial gas was dissolved directly into the magma ocean of the growing planet, where the hydrogen reacted with iron oxides in the mantle to form water. This model predicts that early Earth water initially had a $D/H$ ratio of $0.2 \times 10^{-4}$.

Based on the passage, match each statement about the origin of Earth's water to the model(s) it describes.

A student proposed the following hypothesis regarding the decomposition of hydrogen peroxide ($H_2O_2$) in the presence of the catalyst catalase:

*Hypothesis*: The rate of $H_2O_2$ decomposition increases linearly as the concentration of catalase increases, because more catalyst molecules are available to speed up the reaction.

To test this hypothesis, the student measured the rate of oxygen ($O_2$) gas production (in mL/min) at various catalase concentrations (in percent, $\%$) while keeping the substrate concentration and temperature constant. The results are shown in the table below:

Based on these results, how should the student modify the hypothesis?

### The Younger Dryas Cooling

The Younger Dryas was a period of abrupt cooling that occurred approximately $12,900$ years ago. Two scientists discuss different hypotheses for the cause of this event.

Scientist 1
Around $12,900$ years ago, a massive influx of freshwater from the melting Laurentide Ice Sheet entered the North Atlantic Ocean. Because freshwater is less dense than saltwater, this freshwater remained at the surface, preventing the sinking of warm, salty water that drives the Atlantic Meridional Overturning Circulation (AMOC). The resulting shutdown of the AMOC halted the northward transport of heat, triggering a rapid and severe cooling of the Northern Hemisphere.

Scientist 2
The Younger Dryas cooling was triggered by a comet impact. The comet exploded in the atmosphere, creating widespread fires and sending massive amounts of dust and soot into the atmosphere. This atmospheric debris blocked incoming solar radiation, causing immediate global cooling. The melting of the ice sheet and subsequent freshwater influx into the ocean were consequences of the impact, not the initial trigger of the cooling.

Based on the viewpoints of Scientist 1 and Scientist 2, the scientists disagree on which of the following points?

Europa’s Subsurface Ocean

Jupiter’s moon Europa is covered by a thick ice shell, beneath which a liquid water ocean is believed to exist. Two models propose different mechanisms for how this liquid ocean is maintained.

*Model 1*
Liquid water is maintained primarily by tidal heating. As Europa orbits Jupiter in an eccentric path, the gravitational pull of Jupiter and neighboring moons fluctuates. This variation causes continuous tidal flexing, which generates friction-induced heat within Europa's mantle and ice shell. This heat melts the base of the ice shell, keeping the subsurface ocean liquid. Model 1 assumes that Europa's rocky core contains negligible radioactive elements, meaning radiogenic decay contributes almost no heat to the ocean.

*Model 2*
Liquid water is maintained primarily by hydrothermal activity driven by radiogenic decay in Europa's rocky core. Over billions of years, the decay of radioactive isotopes (such as uranium-235 and potassium-40) in the core has released steady thermal energy. This heat escapes into the bottom of the ocean through hydrothermal vents, keeping the water liquid. Model 2 assumes that Europa's orbit is highly stable and circular, resulting in negligible tidal forces and flexing, and thus tidal heating is insufficient to prevent the ocean from freezing.

Based on the descriptions of the two models, which of the following statements best identifies a primary belief of Model 2 regarding the heat source of Europa's subsurface ocean?

During a biology lab, a student measures the volume of a liquid sample to be $2.5 \times 10^{-3}\text{ liters}$. What is the volume of this sample in milliliters ($\text{mL}$)? (Enter only the numeric value.)

In an environmental science study, a student collects a sample of airborne particulate matter and determines its mass to be $8.5 \times 10^{-6}\text{ grams}$. Which of the following is equivalent to this mass expressed in milligrams ($\text{mg}$)?

Two students discuss the factors that determine the surface temperature of planets orbiting similar stars.

Student 1
A planet's surface temperature is determined solely by its distance from its host star. The closer a planet is to the star, the hotter its surface will be.

Student 2
A planet's surface temperature is determined solely by the thickness of its greenhouse gas atmosphere. The thicker the atmosphere, the hotter its surface will be, regardless of distance.

Match each of the following hypothetical observations of planets to the statement that best describes how the observation aligns with the students' viewpoints.

Experiment 1
Yeast suspensions were incubated in flasks containing $10\%$ solutions of three different sugars (glucose, sucrose, and lactose) at a constant temperature of $35^\circ\text{C}$. The volume of carbon dioxide ($\text{CO}_2$) gas produced was measured every $5$ minutes for a total of $30$ minutes.

Experiment 2
Yeast suspensions were incubated in flasks containing a $10\%$ glucose solution at four different temperatures ($15^\circ\text{C}$, $25^\circ\text{C}$, $35^\circ\text{C}$, and $45^\circ\text{C}$). The volume of $\text{CO}_2$ gas produced was measured only once, exactly $30$ minutes after incubation began.

Statement to evaluate:
The two experiments differed in their independent variables (sugar type vs. temperature) and also in how the dependent variable ($\text{CO}_2$ production) was monitored over time.

### The Xenon Paradox

Compared to chondritic meteorites, Earth’s atmosphere contains only about $10\%$ of the xenon ($Xe$) expected relative to other noble gases, such as krypton ($Kr$) and argon ($Ar$). Two scientists discuss competing hypotheses for this "missing xenon."

Scientist 1
The missing xenon is sequestered in Earth’s deep interior. At pressures exceeding $100\text{ GPa}$ and temperatures above $3000\text{ K}$, which are characteristic of the core-mantle boundary, xenon ceases to be chemically inert. Under these extreme conditions, xenon reacts with iron ($Fe$) and nickel ($Ni$) to form stable intermetallic compounds that sink into the core. In contrast, krypton and argon do not form stable compounds with iron or nickel under core conditions, allowing them to remain in the atmosphere.

Scientist 2
The missing xenon escaped into space early in Earth's history. During the Hadean eon, solar extreme ultraviolet (EUV) radiation was much stronger than it is today. Xenon has a lower first ionization energy ($12.1\text{ eV}$) than krypton ($14.0\text{ eV}$) and argon ($15.8\text{ eV}$). Consequently, xenon was selectively ionized by EUV radiation. The resulting $Xe^+$ ions were dragged out of the atmosphere along with escaping hydrogen ($H^+$) ions driven by hydrodynamic escape. Because krypton and argon remained mostly neutral, they were unaffected by the electromagnetic drag and remained bound to Earth.

Suppose new laboratory experiments demonstrate that at pressures of $120\text{ GPa}$ and temperatures of $3200\text{ K}$, krypton and argon form stable intermetallic compounds with iron and nickel that are just as stable and dense as those formed by xenon. This finding would most directly support or weaken which of the scientists' hypotheses?

### The Origin of Earth's Water

Two scientists discuss the primary source of Earth's water.

Scientist 1
Earth’s water was delivered early in the planet's history by rocky asteroids from the inner solar system, specifically carbonaceous chondrites. These asteroids contain water locked inside their mineral structures. Measurements show that the deuterium-to-hydrogen ($D/H$) ratio of Earth's water matches the $D/H$ ratio found in carbonaceous chondrites, indicating a common origin. Because comets have much higher $D/H$ ratios, they could not have been the main source of Earth's water.

Scientist 2
Earth formed in a region of the solar nebula that was too hot for water to condense, meaning the early Earth was completely dry. Earth's water was delivered much later by icy comets from the cold outer solar system during a period of heavy bombardment. Comets are composed primarily of water ice, making them highly efficient delivery vehicles. While some comets have high $D/H$ ratios, others from the Oort cloud have $D/H$ ratios identical to Earth's ocean water, confirming they were the primary source.

Based on the passage, Scientist 1 claims that Earth's water originated from which of the following sources?

How and when the Grand Canyon was formed is a subject of debate among geologists. Two models propose different timelines and mechanisms for its creation:

Model 1 (Ancient Canyon Hypothesis)
This model proposes that the carving of the Grand Canyon began approximately $70\text{ million}$ years ago ($70\text{ Ma}$). Ancestral river systems slowly carved the canyon over tens of millions of years, driven by the gradual tectonic uplift of the Colorado Plateau. According to this model, the modern Colorado River simply adopted this pre-existing, ancient canyon system.

Model 2 (Young Canyon Hypothesis)
This model proposes that the Grand Canyon is a relatively recent feature, with carving beginning only about $6\text{ million}$ years ago ($6\text{ Ma}$). In this view, several smaller, separate paleocanyons were cut by different rivers over time, but these did not become the Grand Canyon until the modern Colorado River carved through the barriers separating them, integrating the system rapidly within the last $6\text{ million}$ years.

Based on Model 1 and Model 2, the two models differ on which of the following aspects of the Grand Canyon?

### Models of the Early Martian Atmosphere

Two models were proposed to explain the presence of liquid water features on early Mars, despite the young Sun being 30% fainter than it is today.

Model 1 (Warm and Wet Greenhouse Model)
Early Mars possessed a thick, stable atmosphere composed primarily of $CO_2$ and $H_2O$ gas, with a surface pressure of $1.5\text{ to }2.0\text{ bar}$. This thick greenhouse gas envelope was maintained by continuous, global volcanic outgassing. The high surface pressure and potent greenhouse effect raised the average surface temperature above $273\text{ K}$ ($0^\circ\text{C}$), allowing for long-term liquid water oceans and a persistent hydrologic cycle. This model assumes that Mars’s magnetic field was strong enough to protect the thick atmosphere from solar wind stripping during its first 500 million years.

Model 2 (Cold and Icy Impact Model)
Early Mars had a thin, dry $CO_2$ atmosphere with a surface pressure of less than $0.1\text{ bar}$. The average surface temperature was well below $220\text{ K}$, and the surface water was frozen as planet-wide ice sheets. Large meteoroid impacts, which occurred frequently during the Late Heavy Bombardment, delivered transient heat and vast quantities of water vapor. Each major impact event vaporized local ice sheets and injected $H_2O$ and $CO_2$ into the atmosphere, creating a temporary, warm greenhouse effect. Surface temperatures rose above $273\text{ K}$ for periods of only tens to hundreds of years, causing localized, rapid melting and catastrophic flash floods that carved the valley networks before the atmosphere cooled and froze again.

According to the descriptions of the two models, which of the following statements best contrasts the atmospheric pressures and surface temperature dynamics required by Model 1 and Model 2 to explain the presence of liquid water features on early Mars?

### Chemical Reaction Rates

Two students discuss the factors that affect the rate of a chemical reaction.

Student 1
An increase in temperature is the primary driver of increased reaction rates. When the temperature of a reaction mixture is raised, the reactant particles gain thermal energy, which increases their kinetic energy. As a result, the particles move faster and collide both more frequently and with greater force, exceeding the activation energy barrier. Adding a catalyst has no effect on the reaction rate unless the temperature of the system is also increased.

Student 2
Adding a catalyst is the only effective method to increase the rate of a chemical reaction under constant pressure. A catalyst works by providing an alternative pathway with a lower activation energy for the reaction. Reactant particles do not need extra kinetic energy to react because the energy barrier is reduced. Increasing the temperature only increases the thermal energy of the particles but does not alter the rate of the reaction itself.

According to Student 2, a catalyst increases the rate of a chemical reaction by performing which of the following actions?

### Origin of the Grand Canyon

Two geologists present opposing theories regarding how the Grand Canyon in Arizona was formed.

Geologist 1

The Grand Canyon was formed rapidly, over a period of just a few weeks, by a single catastrophic flooding event. Approximately $5,000$ years ago, a large prehistoric lake located northeast of the canyon suddenly breached its natural dam. The sudden release of billions of gallons of water cut through the soft sedimentary rock layers, carving the entire depth and width of the canyon in a very short period.

Geologist 2

The Grand Canyon was formed gradually over millions of years through steady, continuous erosion by the Colorado River. Beginning about $6$ million years ago, the Colorado River began flowing through the region. As the surrounding Colorado Plateau was slowly uplifted by tectonic forces, the river steadily downcut into the bedrock. This slow process of water erosion, aided by wind and weathering, gradually shaped the canyon into its current form.

Based on the passage, Geologist 1 claims that the Grand Canyon was carved by which of the following processes?

During the Paleocene-Eocene Thermal Maximum (PETM), about $56$ million years ago, Earth's global temperature rose rapidly. Two models attempt to explain the source of the carbon release that triggered this warming.

Model 1: A minor initial warming, possibly caused by orbital cycles, warmed the deep oceans. This warming destabilized methane hydrates—solid ice-like structures containing methane gas trapped in marine sediment. Once destabilized, these hydrates dissociated, releasing large amounts of methane gas ($CH_4$) into the ocean and atmosphere, which led to runaway global warming.

Model 2: The rifting of the North Atlantic Ocean caused massive volcanic eruptions. Magma from these eruptions heated organic-rich sedimentary basins, generating and venting carbon dioxide ($CO_2$) and methane ($CH_4$) directly into the atmosphere. The greenhouse effect from these vented gases subsequently warmed the atmosphere and the deep oceans.

Based on these models, is the statement that 'the warming of the deep ocean occurred prior to the release of carbon-containing gases into the atmosphere according to Model 2' true or false?

Eukaryotic cells are distinguished by membrane-bound organelles such as mitochondria and chloroplasts. Two hypotheses propose different models for how these organelles originated.

*Hypothesis 1 (Endosymbiotic Hypothesis)*
Organelles evolved when a large, ancestral anaerobic prokaryote engulfed smaller, specialized prokaryotes. Specifically, aerobic bacteria were engulfed and became mitochondria, while photosynthetic bacteria (cyanobacteria) were engulfed and became chloroplasts. Over time, these engulfed cells formed a symbiotic relationship with the host cell. A key belief of this hypothesis is that organelles contain their own distinct genetic material, which behaves independently of the host's nuclear genome and resembles bacterial DNA.

*Hypothesis 2 (Autogenous Hypothesis)*
Organelles evolved intracellularly through the progressive invagination (infolding) and specialization of the ancestral prokaryotic cell's own plasma membrane. The membrane folds pinched off to form internal compartments that gradually specialized into organelles like mitochondria and chloroplasts. According to this model, the DNA within these organelles is a subset of the cell’s ancestral nuclear DNA, and organelle replication is fully integrated with and controlled by the cell's main nuclear genome.

Based on Hypothesis 2, which of the following statements best describes the origin of the genetic material found within a eukaryotic cell's mitochondria?

A student investigates how different colors of light affect the rate of photosynthesis in *Elodea* plants. The student places one *Elodea* plant in each of four separate glass beakers filled with water. Each beaker is exposed to a different color of light (red, blue, green, or white) by placing colored filters over the light source. To ensure enough light reaches each beaker, the student places the beakers at different distances from the light source: the beaker with the green filter is placed $10\text{ cm}$ away, the blue filter beaker is $20\text{ cm}$ away, the red filter beaker is $30\text{ cm}$ away, and the white light beaker is $40\text{ cm}$ away. After two hours, the student measures the volume of oxygen gas produced by each plant. Which of the following identifies a confounding variable in this experimental design that invalidates the student's conclusion about the effect of light color?

During a biology experiment, a student measures the lengths of four different biological specimens. Based on these measurements, arrange the following specimens in order from smallest to largest length.