Many infectious diseases are becoming difficult to treat because of bacterial resistance to antibiotics. Populations of bacteria can become resistant when they are exposed to an antibiotic. What is the best general explanation for how this occurs?

a) The antibiotic induces specific mutations in some of the bacteria that make them antibiotic-resistant.
b) The antibiotic activates bacterial genes encoding enzymes that can destroy the antibiotic.
c) The antibiotic increases the bacterial mutation rate, so that resistant mutant bacteria are more likely to arise.
d) Antibiotic-resistant mutant bacteria already present in the population survive and reproduce in the presence of the antibiotic.