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.
Consider each of the choices separately and select all that apply.
1. According to the passage, astronomers recognize which of the following as being liable to cause changes to comets?
A. cosmic rays
B. radioactive decay
C. ultraviolet radiation
答案：ABC
Consider each of the choices separately and select all that apply.
2. According to the passage, astronomers’ belief that comets are pristine relics was
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
解析见下。
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.
第1句：研究行星形成的天文学家曾经相信彗星——因为它们大部分留在遥远的奥尔特云，在那里温度接近绝对零度——一定是构成外层行星的材料的原始残留物。
结合后文能看出，这里天文学家觉得彗星是【原始残留物】的原因在于：它们所处的地方（奥尔特云）遥远并且温度低，所以，天文学家们推测奥尔特云那地方变化不大，没有受到什么外界干扰。因此，那里的彗星就能够保留原始形态，不怎么被外界条件改变。既然大部分彗星都在那地方，所以基本上大部分彗星都可以看作维持了原始形态，故而可以叫做“原始残留物”。
这只是天文学家的推测，不一定符合事实。
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.
第2句：从二十世纪七十年代开始，从实验室模拟发现有足够的紫外线辐射到达彗星使它们的表面变暗，并且有足够的宇宙射线来改变（彗星）表面附近的化学键甚至分子结构，将彗星看作原始残留物的概念开始转变。
从二十世纪七十年代开始，老观点被动摇了。因为发现了新证据：
1、实验室模拟发现，有足够的紫外线辐射到达彗星使它们的表面变暗——换句话说，彗星并不是免受外界干扰摧残的，因此不能看成保留了原始状态的【原始残留物】；
2、有足够的宇宙射线来改变（彗星）表面附近的化学键甚至分子结构——和第一点说明的问题一样，还能进一步说明彗星被外界改变的程度更加加深了。第一点只说明表面色泽被改变（变暗），第二点说明连表面的化学键甚至分子结构都能被改变。
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.
第3句：尽管如此，天文学家仍然认为，当一颗彗星靠近太阳时——在那儿他们才可以研究它——太阳强烈的热量会去除腐蚀的表层，从而暴露出内部。
天文学家一般等到彗星到哪儿才能研究彗星？——当它靠近太阳时。
上一句说明，起码彗星表面不能当作【原始残留物】看待：表面已经被宇宙射线、紫外线腐蚀了。
这一句，科学家又给彗星找回了点被研究的价值：虽然表层被腐蚀了，但内部可能还是完好的嘛，那内部还是可以当作【原始残留物】来研究的。
什么时候彗星会露出内部呢？
当它靠近太阳时，太阳强烈的热量会去除腐蚀的表层，从而暴露出内部——这就是研究的好契机。
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.
第4句：然而与此同时，科学家们认识到彗星可能含有会衰退的放射性同位素，这些放射性同位素可能会将彗星的内部温度升高到导致内部进化的温度。
然而，又反转了。这次科学家们又发现，内部也不一定是原始状态。
彗星可能含有会衰退的放射性同位素——这些放射性同位素可能会将彗星的内部温度升高——升高到导致彗星内部进化的温度——彗星的内部可能也改变了。
得，想把彗星作为【原始残留物】来研究看来是没戏了。
Consider each of the choices separately and select all that apply.
分别考虑每个选项并选择所有适用的选项。
1. According to the passage, astronomers recognize which of the following as being liable to cause changes to comets?
根据这篇文章，天文学家承认以下哪一项可能导致彗星变化？
A. cosmic rays
A.宇宙射线
第2句，改变表面。A对。
B. radioactive decay
B.放射性衰变
第4句，decaying radioactive isotopes衰退的放射性同位素，就是改变彗星内部的【放射性衰变】。B对。
C. ultraviolet radiation
C.紫外线辐射
第2句，改变表面。C对。
第二题解析略。需要请留言。