Within six months of effectively using methicillin to treat S. aureus infections in a community, all newS.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 invented and produced drug-resistant proteins 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.

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