When antibiotics were first developed, most infectious diseases could be controlled by them. Today, the diversity of bacteria is increasing, as certain bacteria are resistant to many antibiotics. One possible explanation for this change is that
a
some of the bacteria learned how to resist the antibiotics.
b
the antibiotics killed most of the bacteria that did not have a mutation for resistance.
c
antibiotics have become weaker over the years.
d
the bacteria needed to change in order to produce more antibiotics.