GRE Reading Comprehension
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Source: 2014
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 1018 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.
Question List: 1 2
In the context of the author's argument, the last sentence performs which of the following functions?
- A It explains a criterion that was employed earlier in the argument.
- B It shows that an apparently plausible position is actually self-contradictory.
- C It is a conclusion drawn in the course of refuting a potential explanation.
- D It overturns an assumption on which an opposing position depends.
- E It states the main conclusion that the author is seeking to establish.