Within six months of effectively using methicillin to treat S. aureus infections in a community, all new S. aureus infections were caused by MRSA. How can this best be explained?

A) A patient must have become infected with MRSA from another community.

B) In response to the drug, S. aureus began making drug-resistant versions of the protein targeted by the drug.

C) Some drug-resistant bacteria were present at the start of treatment, and natural
selection increased their frequency.

D) S. aureus evolved to resist vaccines.

Pretty sure it’s C, but I’m not sure.