Why is the answer C?

Hi, the author is trying to prove that the printing press led to a jump in literacy because demand for books jumped after its invention. We are trying to undermine this. C helps in this task because it gives us a reason to think that there was no jump in literacy. If it were true that few new buyers entered the market as C tells us, then we would have no reason to believe that the printing press led to an immediate jump in literacy.