Within six months of effectively using methicillin to treat s. aureus infections in a community, methicillin became less effective. how can this best be explained ?
A. A patient must have become infected from another community.
B. Some drug-resistant bacteria were present at the start of treatment, and natural selection increased their frequency.
C. In response to the drug, s. aureus began making drug-resistant versions of the protein targeted by the drug.
D. The drug aureus that survive methicillin develop an immunity to the antibiotic.