I have been really struggling with this second. But! I figured out in the last lesson (and then applied it to this one) that I have been attacking the wrong part of the argument. So maybe this will help anyone who has been struggling. I have been trying to attack the conclusion and make it invalid. However, I realized that that was not the point. The point is to attack the argument as a whole. So, in other words, attack the system of support. Attack assumptions in the premises; do not attack the conclusions themselves. It sounds simple, but it really did just change the way I was going about answering these questions. I hope this helps someone else who is struggling!

From my understanding, B is not correct because it lacks the "independently owned" qualifier. We are not told in the stimulus that NO pet stores in the town sell gerbils, just none that are independently owned. Answer choice D includes that distinction while B does not.