All practice questions

Two scientists discuss how Earth's surface remained warm during the Archean Eon ($3.8$ to $2.5$ billion years ago), when the Sun's energy output was only 70% to 75% of its current value.

Scientist 1
Atmospheric warming was driven primarily by high levels of carbon dioxide ($CO_2$) and water vapor. $CO_2$ levels were at least 100 to 1,000 times modern pre-industrial levels due to high volcanic outgassing and slow continental silicate weathering. Methane ($CH_4$) played a negligible role because atmospheric oxygen ($O_2$), although low, was sufficient to generate hydroxyl radicals that rapidly oxidized methane, keeping its concentration below $1\text{ ppm}$.

Scientist 2
$CO_2$ alone cannot resolve the warming because geochemical analysis of Archean paleosols (fossil soils) indicates that $CO_2$ levels were constrained to less than 100 times modern pre-industrial levels. Instead, warming was sustained by a mixture of $CO_2$ and $CH_4$. Methanogenic bacteria in the Archean oceans produced massive quantities of $CH_4$, which accumulated to levels above $1,000\text{ ppm}$. This was possible because the atmosphere was virtually anoxic ($O_2$ levels less than $10^{-5}$ of modern levels), preventing methane oxidation.

Suppose a geochemical study of $3.0\text{-billion-year-old}$ marine sediments reveals that the atmospheric $O_2$ level was $10^{-6}$ of the modern level and the atmospheric $CO_2$ level was 40 times the modern pre-industrial level. This finding would most directly support the viewpoint of which scientist, if either?

An agricultural scientist measured the water absorption capacity of three different types of superabsorbent polymer gels (Gel A, Gel B, and Gel C) used in soil conditioning. Six trials were conducted for each gel type, and the mass of water absorbed (in grams per gram of gel) was recorded in the table below:

Based on the table, what is the median water absorption capacity, in grams, for Gel B across the 6 trials?

### Martian Methane Sources

In 2004, sensors detected trace amounts of methane ($CH_4$) in the atmosphere of Mars. Because methane is rapidly destroyed by solar ultraviolet radiation, its presence indicates an active source on or beneath the Martian surface. Three scientists present competing hypotheses regarding the primary source of Martian methane.

Scientist 1
Martian methane is produced by methanotrophic-like microbial life (methanogens) residing in deep, liquid-water aquifers beneath the cryosphere. These microbes utilize carbon dioxide ($CO_2$) and subsurface hydrogen ($H_2$) to produce energy, releasing $CH_4$ as a metabolic byproduct. The hydrogen is generated by radiolysis of water in deep crustal rocks. The seasonal variation in atmospheric methane levels is due to temperature-controlled changes in the permeability of the overlying permafrost, which traps the methane in winter and cracks in summer, releasing the gas.

Scientist 2
Martian methane is abiotic in origin, produced by a geochemical process called serpentinization. Deep underground, olivine-rich rocks react with liquid water and dissolved $CO_2$ at high temperatures (above $150^\circ\text{C}$) to form serpentine minerals, magnetite, and hydrogen gas. In the presence of metal catalysts, this hydrogen reacts with carbon dioxide via the Sabatier reaction to yield $CH_4$. The methane is stored in subterranean clathrate hydrates (icy cages). Micro-fractures caused by tidal stresses from Mars’s moons, Phobos and Deimos, periodically rupture these clathrates, releasing methane plumes into the atmosphere.

Scientist 3
Martian methane is delivered exogenously by carbonaceous chondrite meteorites and interplanetary dust particles (IDPs) falling onto the Martian surface. These materials contain organic macromolecular carbon (carbon polymers). Solar UV radiation photochemically breaks down these organic polymers on the surface of Mars, generating methane gas directly in the atmosphere. The seasonal peak in atmospheric methane coincides with the annual increase in solar UV irradiance during the Martian perihelion (the point in orbit closest to the Sun), which accelerates the photochemical reaction rate.

Based on the hypotheses presented, which of the following statements best describes how Scientist 1 and Scientist 2 differ regarding the role of hydrogen in the production of Martian methane?

Venusian Atmospheric Superrotation

The atmosphere of Venus rotates nearly $60$ times faster than the planet's solid surface, a phenomenon known as superrotation. Two models attempt to explain this phenomenon.

*Model $1$*
Superrotation is driven primarily by solar thermal tides. Solar heating of the dayside atmosphere creates pressure waves (thermal tides) that travel westward, matching the sun's apparent motion. These waves transfer horizontal momentum directly to the upper atmosphere, accelerating the zonal (east-west) winds. This model predicts that wind speeds are highest where solar heating is most intense (at the equator) and decreases significantly toward the poles. Zonal wind speeds are not significantly affected by latitudinal air transport (meridional circulation).

*Model $2$*
Superrotation is driven by a combination of global meridional (north-south) circulation cells and atmospheric eddies. Solar heating causes hot air to rise at the equator and move toward the poles in the upper atmosphere. As this air moves, conservation of angular momentum causes it to spin faster. Eddies (turbulent disturbances) then transport this momentum back toward the equator. This model predicts that meridional wind speeds are critical to maintaining zonal superrotation and that superrotation occurs at all latitudes, with high-speed jet streams forming at mid-to-high latitudes rather than just at the equator.

Based on the models provided, is the following statement True or False?

'Model $1$ supports the belief that the transport of momentum by global north-south (meridional) circulation cells is the primary driver of Venus's atmospheric superrotation.'

To correct a dangling modifier, the subject of the main clause must perform the action described in the introductory phrase. In the sentence 'Walking to school, the rain soaked my jacket,' the introductory phrase 'Walking to school' incorrectly modifies 'the rain.' Complete the corrected version of the sentence by filling in the blank with the correct pronoun.

Consider the underlined portion in the passage below:

In 1977, NASA launched the Voyager 1 spacecraft to study the outer Solar System. Although the spacecraft has traveled farther from Earth than any other human-made object. It continues to transmit valuable scientific data back to Earth.

Which of the following is the best replacement for the underlined portion?

Geothermal heat flux (GHF) beneath the West Antarctic Ice Sheet (WAIS) is a subject of debate among geophysicists. Two researchers propose different explanations for the source and distribution of this geothermal heat.

Researcher 1
Geothermal heat flow is primarily driven by active crustal rifting and mantle magma migration in the West Antarctic Rift System (WARS). GHF is highly localized, exceeding $150\text{ mW/m}^2$ directly above active mantle plumes and fault lines, but dropping below $50\text{ mW/m}^2$ at distances greater than $10\text{ km}$ from these faults. The thickness, age, and mineral composition of the overlying granitic basement rock have no effect on GHF.

Researcher 2
Geothermal heat flow is driven by the decay of radiogenic isotopes ($^{238}\text{U}$, $^{232}\text{Th}$, and $^{40}\text{K}$) within the continental crust. The granitic basement rock beneath the WAIS is thick and highly enriched in these isotopes. Thus, GHF is widely distributed and relatively uniform across the entire region, ranging from $70\text{ to }95\text{ mW/m}^2$. Proximity to active rift faults or tectonic boundaries does not affect GHF.

A study measured the GHF at five borehole locations at varying distances from a major active fault line in the WARS. The granitic basement rock at all five locations has identical thickness and age. The results are shown in the table below:

Based on the information provided, the measured GHF at these locations supports the viewpoint of which researcher, if either?

Although slime molds lack a brain or nervous system, they can solve complex mazes and navigate toward food sources. By leaving a trail of translucent slime as they explore, which serves as a form of externalized spatial memory that prevents them from revisiting areas they have already searched.

Which option corrects the underlined portion to ensure the sentence is grammatically complete?

During a space weather observation, a satellite detector measures the solar wind proton flux. The detector has an active surface area of $4.0 \times 10^{-4}\text{ m}^2$. Over a continuous period of $2.5\text{ hours}$, the detector records a total of $1.8 \times 10^{15}\text{ protons}$. What is the average proton flux measured by the detector, in units of $\text{protons}\cdot\text{cm}^{-2}\cdot\text{s}^{-1}$?

A student investigated the electrical properties of a negative temperature coefficient (NTC) thermistor. The thermistor was placed in a temperature-controlled water bath, and its electrical resistance, $R$ (in kilohms, $\text{k}\Omega$), was measured at various temperatures, $T$ (in degrees Celsius, $^\circ\text{C}$). The data from this experiment are presented in the table below.

Based on the trend shown in the table, what is the predicted electrical resistance, in $\text{k}\Omega$, of the thermistor at a temperature of $110^\circ\text{C}$?

A student conducted an experiment to investigate the effect of pH on the rate of yeast fermentation. The student prepared four flasks, each containing $2.0\text{ g}$ of yeast and $5.0\text{ g}$ of glucose dissolved in $100\text{ mL}$ of distilled water. Each flask was adjusted to a specific pH level and placed in a water bath at a designated temperature. The volume of carbon dioxide ($\text{CO}_2$) gas produced in each flask was measured after $15\text{ minutes}$:

Based on the table, which of the following factors represents a confounding variable that prevents the student from drawing a valid conclusion about the effect of pH on the fermentation rate?

### Faint Young Sun Paradox

During the Archean Eon, approximately 4 billion years ago, the Sun's energy output was only about 70% of its current value. Under these conditions, global temperatures should have dropped below the freezing point of water, yet geological evidence shows that liquid oceans existed. Two scientists present opposing hypotheses to explain this phenomenon.

Scientist 1
The liquid oceans were maintained by a powerful greenhouse effect. Early Earth's atmosphere contained extremely high levels of carbon dioxide ($CO_2$) and methane ($CH_4$). Intense volcanic outgassing released $CO_2$ at rates far exceeding modern levels, while the lack of exposed continental crust limited silicate weathering, a process that removes $CO_2$ from the atmosphere. Simultaneously, early methanogenic bacteria in the anaerobic oceans produced abundant $CH_4$. Together, these gases trapped outgoing infrared radiation, raising surface temperatures enough to prevent global glaciation.

Scientist 2
Greenhouse gases alone cannot explain the liquid oceans because high atmospheric concentrations of $CO_2$ would have led to extensive cloud cover, increasing planetary albedo (reflectivity) and reflecting the limited sunlight back into space. Instead, the primary driver of warming was a low global albedo. Because early Earth lacked continental landmasses, it was almost entirely covered by dark liquid oceans. Since water absorbs far more solar radiation than land or ice, the ocean-dominated planet absorbed a high fraction of the dim solar energy, warming the surface above freezing.

According to the hypothesis of Scientist 2, which of the following conditions was a necessary prerequisite for early Earth to absorb enough solar energy to maintain liquid oceans?

The following passage is about underwater archaeology:

Underwater archaeologists study historical sites submersed in oceans and lakes. These researchers use specialized scuba gear to explore shipwrecks, they must also document their findings with waterproof cameras. Through careful excavation, they preserve history that would otherwise be lost to decay.

Which of the following options is the best revision of the underlined portion?

To track environmental health, acoustic ecologists deploy remote recording devices in diverse habitats. These instruments gather vast amounts of audio data, however, analyzing these recordings requires sophisticated machine learning algorithms. By separating the biophony—the sounds of living organisms—from anthrophony, researchers can track biodiversity trends over decades.

Which of the following options best corrects the underlined portion of the passage?

### Models of Eukaryotic Origin

Eukaryotic cells are characterized by membrane-bound organelles, such as mitochondria and chloroplasts. Biologists have proposed two primary models to explain how these complex internal structures first arose from simpler prokaryotic ancestors.

Model 1 (Autogenous Model)
This model proposes that eukaryotic cells evolved directly from a single ancestral prokaryotic lineage. Through the gradual invagination (infolding) and specialization of the cell's outer plasma membrane, internal compartments formed. This process led to the creation of the nuclear envelope, the endoplasmic reticulum, and eventually mitochondria and chloroplasts. Because all organelles were formed from the host cell's own membrane, this model assumes that mitochondria and chloroplasts should not possess their own distinct genomes, ribosomes, or independent translation machinery.

Model 2 (Serial Endosymbiotic Model)
This model proposes that eukaryotic cells arose through a series of symbiotic relationships between different prokaryotic species. First, an ancestral host cell engulfed free-living aerobic alpha-proteobacteria, which survived inside the host and eventually evolved into mitochondria. Later, some of these early cells engulfed photosynthetic cyanobacteria, which evolved into chloroplasts. Because these organelles were once independent, free-living organisms, the model predicts that mitochondria and chloroplasts should retain their own circular DNA, double membranes, and distinct $70\text{S}$ bacterial-like ribosomes, rather than the $80\text{S}$ eukaryotic ribosomes.

Based on the models provided, match each evolutionary statement or prediction to its corresponding classification.

Avian Migration Navigation

Many migratory birds travel thousands of miles annually, navigating with remarkable precision. Scientists agree that birds utilize Earth's magnetic field, but they disagree on the underlying physiological mechanism.

*Hypothesis 1*
Migratory birds rely on light-dependent chemical reactions in the retina to navigate. Light absorption excites cryptochrome proteins, generating radical pairs (molecules with unpaired electrons). The spin states of these radical pairs are highly sensitive to the orientation of Earth's magnetic field. This chemical signal is processed by the brain as visual patterns of light and shade, allowing the bird to 'see' the magnetic field. Consequently, magnetoreception is impossible in total darkness or under light wavelengths (such as red light) that fail to excite cryptochromes.

*Hypothesis 2*
Migratory birds rely on microscopic crystals of magnetite ($\text{Fe}_3\text{O}_4$), a magnetic mineral, located in specialized sensory cells in the upper beak. These crystals act like tiny compass needles that physically align with Earth's magnetic field lines. This alignment exerts mechanical pressure on cell membranes, opening ion channels and sending electrical nerve impulses to the brain. This mechanism is light-independent and operates equally well in light, total darkness, or under any wavelength of light.

Based on the description of Hypothesis 1, which of the following conditions is necessary for a migratory bird to navigate using Earth's magnetic field?

While kneading the dough for the pizza, a sudden loud noise startled Maria.

Which of the following versions of the underlined portion results in the most logical and grammatically correct sentence?

During an astrophysics study, the density of a stellar nebula's core is determined to be $4.5 \times 10^{-12}$ grams per cubic centimeter ($\text{g/cm}^3$). Which of the following is equivalent to this density expressed in kilograms per cubic meter ($\text{kg/m}^3$)?

The following passage is an excerpt from an essay about avian intelligence.

Crows are renowned for their exceptional problem-solving abilities, often demonstrating cognitive skills on par with those of primates. In a recent study, researchers observed New Caledonian crows fashioning tools out of twigs to retrieve food from deep crevices. A behavior that highlights their capacity for planning and abstract reasoning.

Which of the following options, if any, is the most effective and grammatically correct replacement for the underlined portion of the passage?

Two astrobiologists discuss the origin of methane ($CH_4$) plumes detected in the atmosphere of Saturn’s moon, Enceladus.

Astrobiologist 2
Methane in the plumes is abiotic, formed deep within the core of Enceladus through serpentinization (the reaction of water with olivine rocks) followed by Fischer-Tropsch-type ($FTT$) synthesis. $FTT$ synthesis always produces methane alongside other small alkanes, specifically ethane ($C_2H_6$) and propane ($C_3H_8$). Because $FTT$ synthesis does not significantly fractionate carbon isotopes, the methane in the plumes will have a $^{12}C/^{13}C$ ratio that is nearly identical to the primordial baseline of $89$.

Suppose a new probe measures a plume on Enceladus and detects $CH_4$ with a $^{12}C/^{13}C$ ratio of $108$, but detects no measurable $C_2H_6$ or $C_3H_8$. Based on the provided information, is the statement 'This discovery supports the model of Astrobiologist 2 and contradicts the model of Astrobiologist 1' true or false?