Within a few weeks of treatment with the drug 3TC, a patient's HIV population consists entirely of 3TC-resistant viruses. How can this result best be explained?
(A)HIV can change its surface proteins and resist vaccines. (B)The patient must have become reinfected with 3TC-resistant viruses. (C)HIV began making drug-resistant versions of reverse transcriptase in response to the drug. (D)A few drug-resistant viruses were present at the start of treatment, and natural selection increased their frequency.
D