Why is C better than A?

Hi, I am a bit confused as to why C is correct, and B is not.

A is wrong because the duty is question is that of the buyer. The seller thinks that the buyer acted immorally. That would mean breaching a duty the buyer had. So a sellers duty is not relevant. B is wrong since neither party disputes that the correct amount was given. They dispute the idea that the seller asked for the right price. Both agree the buyer gave the seller the price the sellers asked for. C is right because it tells us the buyers only responsibility is to pay the price asked. This means that since she laid the price asked, she could not have been immoral.m