Veterinarians generally derive some of their income from selling sever

Would an expert be able to weigh in on my reasoning (if it is too extreme) as to why I initially chose Choice (A)? I realize that this question is discussed thoroughly above. Unfortunately, I am still confused.

Given that most of the vets were already selling the manufacturer's products, I was thinking that vets may not want to offer promotions because it may hurt their income by offering promotions (which I understood to be essentially discounting the products... directly cutting into the income that a vet may make on a given product).

For Choice (C):
-I do not understand how just because pet owners are given promotional materials that they now may buy from a supermarket or pet store instead of from their vet. What if the promotional materials were just through buying from the vet/or pet owners decided to buy through the vet because it was the vet (not the supermarket or pet store) who gave the pet owners the promotion?
-I am confused on the Official Guide's reasoning as to why Choice (C) is also correct. The wording is a bit unclear, so I am hoping to break it down. The OG says, "if this manufacturer's products are available in pet stores and supermarkets, but most other products sold by veterinarians are not" does this mean that the vets prefer to sell special products? So, this manufacturer sells a generic toothpaste that is available in many retail channels, but the vets sell a different special toothpaste that you can only buy through your vet?

Then, the OG says, "then distributing the manufacturer's promotional materials could have encouraged customers to buy the manufacturer's products from pet stores and supermarkets". Does this mean that the promotions (whether it be a discount, just plain advertising etc.) would draw a customer's attention to the manufacturer's toothpaste compared to the special toothpaste that the vet sells?

Thank you for bearing through my lengthy questions on this problem