Whoops, just noticed: "a technique called proton-induced X-ray emission" IS a phrase since it has no real verb. I was thinking that it wasn't a CLAUSE. So sorry

Between A and B, A has major pronoun ambiguity ("they") and also ends in a preposition ("for"). When moving a preposition back into the body of a sentence, "for which" is perfectly correct, but it often sounds odd because that's not what we would say in normal speech.

I don't think the problem lies in the " 'S," but more in the comparison. The correct answer states that a black hole lies at the center OF the Milky Way and OF many of the...other galaxies. In this structure, it is then understood that you're talking about the "center" in both &q...

Everything up to "charge" is an independent clause and should be separately appropriately; in this case, a semicolon can be used, eliminating A, B, and C. Logically, this also makes sense since it follows that federal prosecutors would be unlike to pursue infringers while the crime was sti...