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Difficulty: MediumOpening and Concluding Sentences

Oasis in the Abyss

[1]
In 1977, oceanographers aboard the research vessel *Alvin* descended more than a mile into the pitch-black depths of the Galapagos Rift. They expected to find a barren, lifeless oceanic desert, devoid of any significant biological activity. Instead, they made a discovery that revolutionized the scientific understanding of life on Earth: thriving ecosystems surrounding hydrothermal vents. These vents, which are volcanic fissures in the ocean floor, spewed superheated, mineral-rich water from deep within the Earth's crust into the freezing ocean. Surprisingly, massive red-tipped tube worms, ghost-white crabs, and dense clusters of clams flourished in this extreme environment, completely cut off from sunlight.

[2]
Prior to this groundbreaking expedition, scientists believed that all life ultimately depended on photosynthesis, the process by which plants convert sunlight into energy. In the deep sea, however, microbes called extremophiles utilize chemosynthesis instead. Rather than relying on solar energy, these specialized bacteria oxidize toxic hydrogen sulfide gas emerging from the vents to produce organic matter. By doing so, they form the foundation of a unique food web that sustains the larger organisms in the community. This process proved that complex ecosystems could thrive without a single ray of solar energy.

[3]
Consequently, astrobiologists have begun looking at hydrothermal vents as models for potential life on other celestial bodies. If life can flourish in the dark, high-pressure oceans of the Galapagos Rift, similar environments might exist under the icy crusts of Jupiter’s moon Europa or Saturn’s moon Enceladus, where subsurface oceans are believed to exist. ________

The writer is considering how to conclude the passage. Which choice provides the most logical and effective concluding sentence for the passage as a whole?

  1. As space agencies design future missions to explore these distant ocean worlds, the dark depths of our own planet continue to illuminate the possibilities of life across the universe.Answer
  2. B
    These moons are extremely cold and located millions of miles away from Earth, making travel to them highly difficult for human astronauts.
  3. C
    Ultimately, studying these deep-sea vents has redefined the boundaries of biology, it has also expanded our search for life beyond Earth.
  4. D
    Although astrobiologists are excited about Europa and Enceladus; which are moons that might have liquid water underneath their thick ice sheets.

Answer

As space agencies design future missions to explore these distant ocean worlds, the dark depths of our own planet continue to illuminate the possibilities of life across the universe.
The correct option successfully connects the passage's main points about deep-sea hydrothermal vents to the broader search for life in the universe. It transitions smoothly from the discussion of Jupiter's and Saturn's moons in the final paragraph to a reflective concluding statement about how Earth's deep oceans can guide our understanding of outer space. It maintains a formal, objective tone and is grammatically correct.

Step-by-Step Solution

1
Analyze the main theme and structure of the passage.
The passage describes the 1977 discovery of hydrothermal vent ecosystems in the deep ocean, explains how chemosynthetic microbes sustain these communities without sunlight, and suggests that these environments serve as models for finding life on icy moons like Europa and Enceladus.
Understanding the passage's overall flow is necessary to evaluate which concluding sentence best wraps up the text.
2
Evaluate each choice for logical flow, relevance, and tone, ensuring no new unrelated topics are introduced.
The option referring to space agencies designing future missions to explore these distant ocean worlds successfully connects the deep-sea vents to the search for extraterrestrial life, maintaining the passage's scientific tone without introducing off-topic issues.
A good concluding sentence must synthesize the passage's ideas rather than introducing new, unsupported details.
3
Check the remaining options for grammatical and punctuation correctness.
The option containing a comma splice and the option containing a semicolon fragment are grammatically incorrect, leaving only the correct option as both logically fitting and grammatically correct.
ACT English questions require the correct option to be free of grammar and punctuation errors.

Key Concept

Opening and Concluding Sentences
Estimated Time:1m 30s
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Topics

Opening and Concluding Sentences

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