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  3. Astronomers who study planet formation once believed that comets—because they remain mostly in the distant Oort cloud, where tem

Astronomers who study planet formation once believed that comets—because they remain mostly in the distant Oort cloud, where tem

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问题     Astronomers who study planet formation once believed that comets—because they remain mostly in the distant Oort cloud, where temperatures are close to absolute zero—must be pristine relics of the material that formed the outer planets. The conceptual shift away from seeing comets as pristine relics began in the 1970s, when laboratory simulations revealed there was sufficient ultraviolet radiation reaching comets to darken their surfaces and there were sufficient cosmic rays to alter chemical bonds or even molecular structure near the surface. Nevertheless, astronomers still believed that when a comet approached the Sun—where they could study it—the Sun’s intense heat would remove the corrupted surface layer, exposing the interior. About the same time, though, scientists realized comets might contain decaying radioactive isotopes that could have warmed cometary interiors to temperatures that caused the interiors to evolve.
According to the passage, astronomers’ belief that comets are pristine relics was
选项 A、overturned by analysis of what happens when comets approach the Sun
B、supported by what observations revealed about the composition of the outer planets
C、based on consideration of the conditions that prevail where comets are located
答案C
解析 彗星所在地方的普遍环境条件指的是第一句中的绝对零度的奥尔特星云，prevail对应mostly，因此选项C正确。A项对彗星接近太阳时所发生的事情的分析对应第三句，该句支持了彗星是纯洁的这一观点。B项彗星纯洁与否和外部行星的组成无关。
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