2. 外语
  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 recognize which of the following as being liable to cause changes to comets?
选项 A、Cosmic rays
B、Radioactive decay
C、Ultraviolet radiation
答案A,C
解析 本题问的是天文学家的观点，因此定位到一三两句。第一句认为彗星是纯洁的，排除。第三句天文学家承认了第二句彗星不纯洁的理由，包含紫外辐射和宇宙射线，因此选项A、C正确。B项干扰选项，定位在本文最后一句话，这句话是科学家说的，不是天文学家的观点。
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本试题收录于： GRE VERBAL题库GRE分类
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