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Difficulty: HardEstablishing Introductions and Thesis Statements

The following is an excerpt from an essay about early Earth's climate.

[1]
In 19721972, astronomers Carl Sagan and George Mullen pointed out a puzzling contradiction that threatened our understanding of Earth's early history. According to standard astrophysical models, our sun was about 3030 percent dimmer during the Archean Eon, four billion years ago, than it is today. With so little solar energy reaching the planet, Earth should have been a frozen wasteland, completely locked in ice. Yet, geological evidence from that same period—such as water-worn pebbles and ancient sedimentary rocks—reveals that liquid water flowed abundantly across the surface, and primitive life was already thriving.

[2]
To resolve this paradox, researchers have proposed various atmospheric models. One prominent theory suggests that a dense greenhouse gas envelope, consisting of carbon dioxide and perhaps ammonia or methane, acted as a heavy blanket to trap what little heat the faint sun provided. Others argue that Earth's lower albedo, or reflectivity, allowed the surface to absorb more solar radiation than it does today. Understanding how Earth maintained its liquid oceans during this period is not just a historical curiosity; it also helps astronomers define the boundaries of habitable zones around distant stars.

Which choice provides the most effective transition from the first paragraph to the second paragraph and introduces the main topic of the second paragraph?

  1. A
    Although scientists wanted to resolve this paradox; they proposed various atmospheric models.
  2. To resolve this paradox, researchers have proposed various atmospheric models.Answer
  3. C
    Although many scientists were shocked by this contradiction and wanted to find a solution.
  4. D
    To resolve this paradox, researchers proposed various atmospheric models, they study gases.

Answer

The phrasing that states 'To resolve this paradox, researchers have proposed various atmospheric models.' is correct because it provides a grammatically sound transition that links the paradox of the first paragraph to the scientific models described in the second paragraph.
The correct answer, which mentions proposing various atmospheric models to resolve the paradox, is grammatically correct and establishes a logical link between the paradox of the faint young sun (described in the first paragraph) and the specific atmospheric hypotheses (discussed in the second paragraph).

Step-by-Step Solution

1
Analyze the relationship between the first and second paragraphs to determine the transition needed.
The first paragraph describes the 'faint young sun paradox' (a dim early sun but liquid water on Earth), while the second paragraph discusses specific theories (greenhouse gases and albedo) to explain it.
An effective transition must connect this contradiction (the paradox) to the explanations (the models).
2
Evaluate the grammar of the transition options.
Identify that several options introduce grammatical errors such as sentence fragments, comma splices, or incorrect semicolon usage.
The correct option must not only be contextually appropriate but also grammatically correct.
3
Identify the correct choice based on grammar and context.
The correct choice connects the paragraphs logically without introducing any punctuation or structural errors.
This ensures the passage maintains flow, clarity, and grammatical precision.

Key Concept

Establishing transitions and paragraph-level topic sentences that logically connect and introduce paragraph topics.
Estimated Time:1m 15s
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Topics

Establishing Introductions and Thesis Statements

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