求教一道短阅读题

二零一五劳动节UID: 3674708

来自Verbal150题

Supernovas in the Milky Way are the likeliest source for most of the cosmic rays reaching Earth. However, calculations show that supernovas cannot produce ultrahigh-energy cosmic rays (UHECRs), which have energies exceeding  electron volts. It would seem sensible to seek the source of these in the universe’s most conspicuous energy factories: quasars and gamma-ray bursts billions of light-years away from Earth. But UHECRs tend to collide with photons of the cosmic microwave background—pervasive radiation that is a relic of the early universe. The odds favor a collision every 20 million light-years, each collision costing 20 percent of the cosmic ray’s energy. Consequently, no cosmic ray traveling much beyond 100 million light-years can retain the energy observed in UHECRs.

In the context of the author’s argument, the last sentence performs which of the following functions?
A explains a criterion that was employed earlier in the argument.
B shows that an apparently plausible position is actually self-contradictory.
C is a conclusion drawn in the course of refuting a potential explanation.
D overturns an assumption on which an opposing position depends.
E states the main conclusion that the author is seeking to establish.

答案：C
解析：The last sentence is the conclusion of an argument in the last half of the passage; it puts a constraint on the possible origin of UHECRs relative to Earth and thereby rules out the possibility, mentioned earlier in the passage, that distant quasars and gamma-ray bursts could be the origin of UHECRs. Therefore Choice C is correct. It is important to note that the last sentence does not show any plausible position to be self-contradictory (Choice B), and that it does not state the author’s main conclusion (Choice E), since it is relevant to only one of the two hypotheses considered in the passage.

问题：我能读懂文章和题干，知道最后一句话的作用是为了反驳之前提出的UHECRs可能源自quasars and gamma-ray bursts billions of light-years away from Earth。但我不太能区别选项间的差异，一开始做的时候觉得BCD好像都差不多（contradictory, refuting, overturns）好像都是结尾逆转，最后蒙了一个B。看过解析后，理解C为什么对，懵懵懂懂感觉B可能是错在了self-contradictory，但是还是不理解D，从中文上我都不大能翻译出D选项的准确含义。

请忙我分析一下D选项好吗？谢谢大家！