The only way that bookstores can profitably sell books at below–market prices is to get the books at a discount from ...

This one confuses me. It would seem that D could be true. To generate volume (to get the discount) the book store must EITHER cater to mass tastes OR have exclusive access. D argues they don't have exclusive access but still profitably sell books at a discount. Since it seems there are 2 options-(OR) isn't it "possible" that they could get high volume by catering to mass tastes and thereby getting a high volume and thereby get the discount? The contrapostive states both NOT catering to masses and NOT having exclusive access but D says nothing about catering to the masses? Thank you!