The phenomenon we want to explain is that the actual drug prices are much lower than the pharmaceutical guidebook recommends. Since this is an "except" question, each of our wrong answers is going to give a possible explanation. Our correct answer (B), doesn't explain it because the stimulus states "for the same drugs."
You asked about D, which means you don't see how it explains the disparity between actual prices and recommended prices. Here is the problem. We do not know if the wholesalers are making substantial profits. They could be making small profits, or have thin margins. If the guidebook is calculated to generate substantial profits, then a wholesaler could have lowered prices to undercut the competition, as seen in answer choice A. We do not have to be certain that D is the reason for the disparity, but we can say that it is a possible reason.
