Explanation

1) All three choices are correct. The question asks about claims the author would agree with. Choice A is correct: The passage states that supernovas in the Milky Way “cannot produce ultrahigh-energy cosmic rays.” Choice B is correct: Since very distant quasars and gamma-ray bursts are
“billions of light-years away from Earth,” they are too far away for a UHECR to reach Earth. Choice C is correct: The last sentence of the passage states that “no cosmic ray traveling much beyond 100 million light-years can retain the energy observed in UHECRs.”

2) 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.
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The first question's third choice seems ambiguous as the passage points out much beyond than 100 million light years, however, the answer choice (C) says no more than 100 million light years. How that can be?
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Yes, I totally agree with you if not for a small word you forget to mention in that phrase.



with this in the passage

Consequently, no cosmic ray traveling much beyond 100 million light-years can retain the energy observed in UHECRs.

They perfectly match.

Hope this helps

Regards
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Thank you, the test organizers are so cunning. Also, what you think my level is if I make such a mistake? Thanks in advance.
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Difficult to judge based on one question only. It could be a silly mistake or something related with your English skills.
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Can anyone help me to understand the 2nd question, please? it seems that the answer choice B is correct. I have been thinking why C is correct and it seems ok to be the right answer but I just can't understand why B is not the correct one. Because to me, it feels like quasar and gamma rays were plausible position for the cosmic ray's source and the last line was contradictory to this explanation for the source of those cosmic rays. I just can't find my mistake.

Venomous passage

Analyzing it as the following:

X (Supernovas) are composed by \(x_1,x_2,\) ......so forth x-rays type

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.

However, our X does not produce a CERTAIN type of rays which can not reach our planet BECAUSE 20% of those rays are lost every 20 millions of years. In 100 million light-years they become zero, basically.

Therefore our conclusion is the following

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) It explains a criterion that was employed earlier in the argument.

No. It is a conclusion not an explanation. it is the result

B) It shows that an apparently plausible position is actually self-contradictory.

It does not contradict anything before

C) It is a conclusion drawn in the course of refuting a potential explanation.

Yes. It is. Our potential explanation is that some other source is the origin of our rays. And the last sentence concludes that the rays from supernovas can not reach or pass over 100. The explanation is the match calculation above

D) It overturns an assumption on which an opposing position depends.

Nothing here is assumed

E) It states the main conclusion that the author is seeking to establish


The author does not establish anything. We have specific facts. Basically E says that we have an assumption but we do not. D and E says the same thing
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quite a tough one!
Thanks for posting it!