Why not B

I found the answer, the merchants have nothing to do with the economists who's belief has being discussed.

@Alyona1983, Let's take a look at (B). (B) says, "Few merchants of any kind have detailed knowledge of economic theories about the relationship between item prices and sales rates." The reason we can get rid of (B) is because it does not matter whether or not the merchants know lots of economic theories. It does not require that level of knowledge to know how much wine you're selling and what type of wine you're selling, so this is why (B) is out. Let us know if you have any other questions!