Yes, B is the correct answer overall. But, I still think a case can be made for A. After all, even though your explanation for not choosing A is that there is no way of knowing for certain whether the quagga would have survived, you also say that humans were responsible for the animals' extinction. I am not necessarily an animal rights spokesperson; however, there is an understandable assumption that the species' survival would have been more likely had they not been hunted by people. The paragraph should have added a sentence which, however indirect (so as not to clearly state something that would provide a clue to the examinee), would shed a bit more light on this issue.

One of the issues I take with some of these standardized tests in Reading Comprehension is that, while it's proper for students of reading to not make assumptions about subject matter conclusions, often the writer expresses ideas that are often ambiguous. Not all teachers teach their students to think deeply critically about things. Often, it's just an issue of time. And frankly, writers, themselves, often write in ways that are deliberately meant to allow for a certain liberal or "open ended" interpretation by the reader. Not all academic writers or non-fiction writers compose thoughts as if they were writing a thesis or dissertation.