I am confused with the explanation given

sameeeeeeeeeeeeeee

When I was looking at the answers, I chose C over E because the "unstated assumption that some patients who take penicillin develop bacteria with an immunity to penicillin" does not "prove" anything as answer E suggests, it merely poses the question that the passage tries to answer. Choice C describes it by saying it "generates the problem" for the passage, which is what I thought it did.