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 can be inferred from the passage about methanogens?
选项 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E
解析 根据最后一句，蓝藻的出现导致甲烷菌减少，因此如果甲烷菌出现在蓝藻之后，可能根本就没有甲烷菌存在了，也就无法给地球产生影响，所以选项E正确。A项甲烷菌应该出现在23亿年至46亿年前之间。B项文中未提及流行程度。C项文中未提及甲烷是否容易受到紫外光线影响。D项甲烷菌厌氧，但是氨气如果没有氧气就会被破坏，因此没有氨气的条件也意味着没有氧气，甲烷菌是可以生存的。
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