Official Explanation
OK, for simplicity, I will say R = (Radon-210), T = (Technetium-99), PM = (Promethium-61) and PN = (Protactinium-234).
The argument has the premises
(i) R > T
(ii) PM > PN
and it wants to conclude R > PN.
This is an EXCEPT question. Four of the five responses will, when added to premises (i) & (ii), make the entire argument valid: these four will be incorrect. One of the answers will either contradict the conclusion or leave open uncertainty, and this oddball answer will be the correct answer to this question.
The credited answer is (D), which says PM > R. This means both R & PN are less than PM, but we are left with uncertainty about how R and PN compare to one another. Choice (D) leaves us with uncertainty, so it is correct answer to the EXCEPT question.
Choice (A) says T > PM. That means R > T > PM > PN, which directly implies that R > PN. Choice (A) turns this into a valid argument supporting the conclusion, so it is incorrect.
Choice (B) says T = PM. That means R > T = PM > PN, which directly implies that R > PN. Choice (B) turns this into a valid argument supporting the conclusion, so it is incorrect.
Choice (C) says PM = R. That means R = PM > PN, which directly implies that R > PN. Choice (C) turns this into a valid argument supporting the conclusion, so it is incorrect.
Choice (E) says T > PN. That means R > T > PN, which directly implies that R > PN. Choice (E) turns this into a valid argument supporting the conclusion, so it is incorrect.
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