S . aureus diagnoses were brought on by MRSA six months after methicillin was successfully used to treat S . aureus outbreaks in a community. What is the easiest way to understand this?

A. The vaccine was not able to identify the target species.
B. The vaccine used contains a stronger strain of bacteria.
C. Some S . aureus bacteria were able to survive and had evolved to a more resistant type.
D. Another source of infection had sprung.