Why B?

The argument is about eliminating poaching with the end goal of saving the species, as spelled out in the conclusion. If we prevent poaching by doing something that incidentally dooms the species, then we failed to achieve that end goal.

But there is no indication in the passage that it would harm them in such a way. In fact, there's no information in the passage that suggests horns have anything to do with mating. So why is it "required" for the argument to work? At best, it seems to be an answer to an objection that no one raised.