Explanation
In this question, as in any question that involves
weakening an argument, such as many Critical Reasoning questions, the correct
answer will most likely be one that does more than argue against the conclusion
the argument; it will most likely attack the connection between the evidence
and the conclusion, as such a connection is the essence of any argument. We can
consider each answer choice in turn to be true and see how seriously it damages
the argument. Choice (A) sounds problematic until you notice it refers to supermassive black holes, not just black
holes, so (A) is out. Choice (B) is not a problem; it's a necessity, because by
definition the accretor is more massive (line 22).
Choice (C) is somewhat nonsensical, because the X-ray emissions themselves are
not attributed to black holes; they
originate with the stream of mass that comes out of the smaller member of the
binary, the non-black hole. So we are down to (D) and (E). To evaluate (D),
let's see what the lower end of the range estimate for the mass of Cygnus X-1
is. It's "14 solar masses, plus or minus 4 solar masses" (lines 37-38). The
lower end of that range is 10 solar masses. Would it be an issue if the black
hole candidates are all in this area? In fact, 10 solar masses is the
alternative upper limit for the Tolman-Oppenheimer-Volkoff limit, if "more generous assumptions are made"
(lines 43-45). So that could be a problem, because in that case the stars might
not be conclusively above the limit. So (D) could be the correct answer. On to
(E), which states that most accetors in binary
systems are known not to be black holes. Is this a problem? It's not a problem
if they have masses below the limit, which may, in fact, be the case. So (E) is
out.
The correct answer is (D).
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