What could happen if Susan stopped taking her antibiotics when she felt better, instead of taking them for the length of time directed by her doctor?

a
The bacteria could turn into a superbug, as it builds up a resistance to the antibiotic over time.
b
The bacteria could be destroyed by the antibiotic, and that particular strain would never appear again.
c
The bacteria could cause her immune system to build a resistance to that strain of bacteria forever.
d
The bacteria could cause her immune system to stop producing the antibodies to fight any strain of bacteria.

a. The bacteria could turn into a superbug, as it builds up a resistance to the antibiotic over time.