An antibiotic is given repeatedly to treat a recurrent ear infection. It worked initially but now is no longer effective. This indicates that the Streptococcus bacterium

A) is being treated with an antibiotic that has experienced a manufacturing error, making it infective.
B) is very sensitive to the antibiotic.
C) is infectious only if it is sensitive to the antibiotic.
D) experienced natural selection that has allowed the resistant Streptococcus to survive and multiply.