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.
The author suggests that the realization described in the final sentence of the passage had which of the following  effects?
选项 A、It introduced a new topic for study by astronomers interested in planetary formation.
B、It led astronomers to adopt a number of different strategies in trying to determine the composition of cometary interiors.
C、It called into question an assumption that astronomers had made about comets.
D、It cast doubt on astronomers’ ability to study the interior parts of comets.
E、It caused astronomers to revise their account of the composition of the outer planets.
答案C
解析 本题属于功能题，正确答案是定位句3s版本。由最后一句的句间关系可知，该句的功能在于反驳前一句，因此选项C正确。A项无关选项。B项无关选项。D项最后一句反驳的是前一句天文学家的观点，而非天文学家的能力。E项最后一句只是反驳了天文学家的观点，没有使天文学家修正自己的观点。另外，文章讨论的是彗星是否纯洁，这和外部行星的组成无关。
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本试题收录于： GRE VERBAL题库GRE分类
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