<p>The CB explanation is sort of correct. Choice C does contain a comma problem. The rule is that, if you have an adverb clause <em>after</em> the main clause, you don't need a comma. If you have an adverb clause <em>before</em> the main clause, then you do need a comma. So you'd write, "Since I was hungry, I ate a sandwich," but "I ate a sandwich since I was hungry." It's technically incorrect to put a comma in the second sentence, although expert prose writers make some version of this "error" all the time.</p>
<p>Nonetheless, I find CB's explanation poor. The comma is the least of the problems with choice C. Take a look at choice C with the comma problem corrected:</p>
<p>"The fruit fly is often used to study genetic mechanisms since, with its rapid reproduction, scientists can observe the effects of experiments on several generations."</p>
<p>Wha. . . ? Just what function is the word "with" performing in this sentence? Are the scientists observing their experiments "with the rapid reproductions of the fruit flies"? What would that even mean?</p>
<p>Presumably, the word "with" is being used here to tell us why something is true: it's a synonym for "because of." But "with" cannot directly express cause-and-effect relationships like this.</p>
<p>The reason that choice C might sort of sound okay to you is that the word "with" can very rarely be used to form a phrase that tell us <em>under what circumstances</em> the main clause is taking place:</p>
<p>"With all the rain, they had to move the party indoors" = "Under the circumstances of the rain, they had to move the party indoors."</p>
<p>At first glance, it seems that the phrase beginning "with" is just giving a reason for the main clause, but its function is a little bit more complicated than that. <em>Under what circumstances</em> is not always the same thing as <em>why": in fact, it usually isn't, and it isn't in choice C. The rapid reproductions of the fruit flies are not a *circumstance in which</em> the scientists can observe their experiments, so we can't use "with."</p>
<p>I'm going in to depth about the correct adverbial use of "with" here because I've seen it tested more than once on real CB exams (although never adequately addressed by whoever is writing the CB explanations).</p>
<p>This is something I've noticed before with CB explanations: they are usually not wrong, but they are often not that helpful, either, and they often don't go to the heart of the wrong answer choice. I don't know who's writing them, but I don't think it's a linguist or a usage expert.</p>