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On a college campus, health services began using methicillin to treat S. aureus bacterial infections in its athletes. Within six months of starting this effective treatment method, 85% of all new infections were caused by methicillin-resistant S. aureus organisms.

Which of the following BEST explains this phenomenon?


Some drug-resistant bacteria were present at the beginning of the new treatment program, and the addition of the drug methicillin changed the environment so that natural selection increased their frequency.


S. aureus altered the structure of the protein targeted by methicillin in response to the drug's presence.


A student on the campus must have contracted a methicillin-resistant strain of S. aureus during an away game at another college.


S. aureus is unaffected by vaccines and can only be treated by antibiotics.

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