GRE阅读真题备考——第二十三篇

　Passage 23

　　When Earth formed 4.6 billion years ago, the Sun burned only 70 percent as brightly as it does today. Yet Geologic record contains no evidence for widespread glaciation until 2.3 billion years ago. Sagan and Mullen suggested in the 1970s that ammonia, a greenhouse gas, warmed early Earth’s atmosphere, but subsequent research showed that the Sun’s ultraviolet rays rapidly destroy ammonia in an oxygen-free environment, such as that of early Earth. Many scientists now attribute much of the warming of early Earth to oxygen-intolerant microbes—methanogens—that produce the greenhouse gas methane. The methanogen hypothesis could help to explain the first global ice age: 2.3 billion years ago, Earth’s atmosphere began to fill with oxygen produced by other microbes—cyanobacteria—causing methanogens to decline rapidly.

　　1. Which of the following best describes the function of the highlighted sentence?

　　A. It refutes the possibility of a connection between two events previously thought to be related.

　　B. It describes how a hypothesis might account for the timing of a phenomenon described earlier in the passage

　　C. It presents evidence that casts doubt on a statement made in the first sentence of the passage

　　D. It clarifies a distinction between two related hypotheses

　　E. It introduces findings that challenge a dominant explanation for a particular phenomenon

　　2. Which of the following can be inferred from the passage about methanogens?

　　A. Methanogens must have appeared on Earth later than 2.3 billion years ago.

　　B. Methanogens must have been much more prevalent in some regions of the early Earth than in others.

　　C. Methanogens produce a greenhouse gas that is more susceptible to destruction by the Sun’s ultraviolent rays than is ammonia.

　　D. Methanogens could not have thrived in early Earth’s atmosphere without the presence of ammonia.

　　E. Methanogens would have had a less significant effect on early Earth’s atmosphere if they had evolved after the appearance of cyanobacteria.

　　答案：B E

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