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 a resistant virus. c. A few drug-resistant viruses were present at the start of treatment, and natural selection increased their frequency. d. HIV began making drug-resistant versions of its enzymes in response to the drug.