Explanation
This question looks easy at first: an accretor
in an X-ray binary with a mass above the limit is probably a black hole. But,
on turning to the answer choices, we see that our knowledge only narrows the
answer choices by one. We will have to get more specific and eliminate answer
choices on the basis of what is stated in the passage. Looking back, we can see
that the mass of these things above 3.6 and up to 14, but not automatically
above 10, so both choices (B) and (C) are too restrictive. Is the thing moving
or stationary? It's moving; that's how the calculation of binary stars work and
it's the main subject of the beginning portion of the second paragraph: they
rotate around their "common center of mass," and since this is "a point between
them" (line 20), they must both necessarily be in motion.
The correct answer is (D).
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