GRE阅读题目解析:46亿年前太阳的亮度

　　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.

　　1

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

　　46 亿年前地球刚形成时，太阳燃烧发出的亮度值相当于今天的七成。

　　2

　　Yet Geologic record contains no evidence for widespread glaciation until 2.3 billion years ago.

　　然而地质记录中直到 23 亿年前才首次出现广泛的结冰期的证据。

　　3

　　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.

　　S 和 M 于 1970 年代提出氨，一种温室气体，导致地球大气暖化，但随后的研究表明，太阳紫外线在无氧环境中快速分解了氨，这正如形成初期的地球的大气环境。

　　4

　　Many scientists now attribute much of the warming of early Earth to oxygen-intolerant microbes — methanogens — that produce the greenhouse gas methane.

　　现在，许多科学家把早期地球暖化，主要归因于厌氧微生物 —— 甲烷菌 ——它产生温室气体甲烷。

　　5

　　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.

　　甲烷菌假设，可以帮助解释第一次全球结冰期：23 亿年前，地球大气开始充满其他微生物 —— 藻青菌生产的氧气 —— 致使甲烷菌迅速减少。

　　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

　　选 B

　　它描述了一种假设，也许可以解释前文出现的一种现象出现的时机。

　　文段开头描述了一种现象：地球刚开形成时，太阳燃烧发光比现在弱(暗示那时地球应该很冷)，但地址记录中直到地球形成 23 亿年后，才出现第一次广泛冰期的证据(意味着最开始的 23 亿年并不冷)。

　　高亮句对这种现象给出一种假设，作为解释：

　　最开始地球上充满了 methanogens，它们讨厌氧气，它们每天的工作就是产生甲烷，而甲烷作为一种温室气体，使地球在太阳照射较弱的前 23 亿年保持温暖。但 23 亿年的某个时间，突然出现了大量的 cyanobacteria 藻青菌，这些家伙开始生产氧气，氧气毒死了之前那些 methanogens，也就减少了大气中的甲烷 methan，结果地球变冷，出现了第一次冰期。

　　所以选 B 。

　　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.

　　选 E

　　从文中可以得到什么推论。

　　A

　　产甲烷菌一定出现在 23 亿年以后。错的离谱。文段逻辑显然是，前 23 亿年无冰期都靠 methanogens 撑场面。

　　B

　　产甲烷菌在某些地区更多。未知信息。

　　C

　　产甲烷菌生产一种，比氨更容易被太阳紫外线摧毁的温室气体。说反了。

　　D

　　错。因为 methanogens 就是产甲烷的，从名字都能看出来。

　　E

　　如果 methanogens 比 cyanobateria 出现的晚，作用就不那么重要了。正确，因为后者会杀死前者，前者的暖化作用就被削弱了。

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