2. 外语
  3. When Earth formed 4.6 billion years ago, the Sun burned only 70 percent as brightly as it does today. Yet geologic record contai

When Earth formed 4.6 billion years ago, the Sun burned only 70 percent as brightly as it does today. Yet geologic record contai

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问题     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.
Which of the following best describes the function of the highlighted sentence?
选项 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
答案B
解析 本题属于功能题，根据句间关系可知这句话用于支持前一句，即甲烷菌假说可以解释地球的冰川现象发生时间，因此选项B正确。a hypothesis对应The methanogen hypothesis，a phenomenon指第二句的widespread glaciation，timing指的是23亿年之前。A项本文唯一可以用作反驳功能的，只有第三句的but部分。C项第一句呈现的是事实，而非观点，无法被质疑。D项这句话仅仅在解释甲烷菌假说本身，没有涉及之前萨根和马伦的假说。E项这句话用来支持而不是质疑甲烷菌假说，而且a dominant explanation在文中没有对应。
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本试题收录于： GRE VERBAL题库GRE分类
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