Lice are parasitic insects that live on the scalp, eyebrows, and eyelashes of humans
and feed on blood. Although there have been a variety of cures throughout history,
treatment for lice has changed very little since 1945, when pyrethrum, a nontoxic
insecticide, was developed. In recent years, however, a strain of "super-lice" has
become resistant to these over-the-counter treatments used to kill lice. Which of the
following best describes how the treatments have become less effective on the lice?

A. Some lice had a gene mutation that helped them survive treatments and
reproduce, passing on this helpful trait.

B. Lice have been selectively bred to become stronger and resistant to the
treatments over time.

C. Individual lice grew a protective exoskeleton that prevented them from coming
into contact with the treatment.

D. The treatments have been diluted, making the insecticide weaker and allows
more lice to survive.