How Could Some of the Original Strain of S

How could some of the original strain of S.aureus bacteria survive in the presence of methicillin?

A) The methicillin caused mutations in the bacteria
B) They had pre-existing mutations that gave resistance
C) The methicillin was no longer active
D) The methicillin may not have been added to those plates
E) The bacteria wanted to survive the antibiotic and mutated to become resistant