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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.
It can be inferred that the author of the passage would agree with which of the following about the origin of UHECRs that reach Earth?
A.The origin is something other than supernovas in the Milky Way.
B.The origin is most likely something other than very distant quasars or gamma-ray bursts.
C.The origin is most likely no more than a little over 100 million light-years away from Earth.
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.
