Yeah B is not correct because catering to mass tastes or having access to large market is necessary but not sufficient. However, I'm a little confused with C. From the passage, I understand that getting books at a discount from publishers is necessary for profitably selling books at below-market prices. But C is saying the other way around; profitably selling books at below-market prices will get you a discount. It would be clearer if C were " bookstore that profitably sells books at below-market prices RECEIVED discounts from publishers.