Isn't it possible that most of the species not hunted were also not encountered by the migrating groups? In that case the author could simply say there was no way for the micro organisms to infect then, therefore they didn't go extinct.

Couldn't you could skim the answer choices for the phrase "most productive" and that would have to be the right answer as long as they're a programer since the way each rule treats that subset contradict?

For answer choice B, is it generally safe to assume that whenever a word like "some" or "most" is modifying an adjective (in this case, "common") instead of a noun (in this case "species"), that there is no set-based logic present?