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Question

Difficulty: MediumEstablishing Introductions and Thesis Statements

[1]
__________________________________________________ Before 1977, scientists believed that all marine life ultimately depended on sunlight, which powers photosynthesis in surface waters. However, when the research submersible Alvin descended to the ocean floor near the Galápagos Rift, researchers discovered thriving ecosystems clustered around vents spewing mineral-rich water at temperatures up to 750°F. Rather than depending on solar energy, these communities relied on chemosynthetic bacteria, which convert toxic hydrogen sulfide into organic matter.

Which of the following choices best introduces the paragraph by establishing its main focus?

  1. The discovery of deep-sea hydrothermal vents revolutionized our understanding of the biological boundaries of life on Earth.Answer
  2. B
    Because the deep ocean floor was long considered a barren wasteland devoid of light and heat.
  3. C
    Oceanographers have spent decades exploring the deep sea, they still have much to learn about these underwater environments.
  4. D
    The initial planning for the deep-sea expeditions to the Galápagos Rift were focused primarily on geological research rather than biology.

Answer

The option stating that the discovery of deep-sea hydrothermal vents revolutionized our understanding of the biological boundaries of life on Earth.
The correct option successfully introduces the main focus of the paragraph by mentioning the discovery of deep-sea hydrothermal vents and the shift in biological understanding. This sets up the context for the next sentence, which explains what scientists previously believed about life needing sunlight, and how the discovery of hydrothermal vents challenged that assumption.

Step-by-Step Solution

1
Analyze the passage to identify its main idea and supporting details.
The passage discusses the discovery of hydrothermal vents near the Galápagos Rift and how the ecosystems around them survive via chemosynthesis rather than photosynthesis.
An effective introduction must set up the topic and main argument of the paragraph that follows.
2
Evaluate each option for grammatical correctness.
The option starting with 'Because' is a fragment. The option stating 'Oceanographers have spent...' is a comma splice. The option starting with 'The initial planning...' has a subject-verb agreement error ('planning ... were'). Only the correct option is grammatically sound.
ACT English questions require sentences to be grammatically correct in addition to rhetorically effective.
3
Evaluate the grammatically correct options for rhetorical effectiveness.
The statement about hydrothermal vents revolutionizing our understanding of biological boundaries directly introduces the contrast with the historical belief about sunlight-based life.
The correct option must logically connect to the subsequent sentences, which contrast previous scientific beliefs with the new discovery.

Key Concept

Establishing Introductions and Thesis Statements
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Topics

Establishing Introductions and Thesis Statements

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