3TC is an antiretroviral treatment used to prevent HIV infection from becoming AIDS. After treatment with 3TC, the HIV population in a patient consists entirely of 3TC-resistant virus particles. What best explain this phenomenon? A) HIV can change its surface proteins through mutations and resist vaccines. B) The patient became re-infected with 3TC-resistant viruses after the initial exposure. C) The drug caused RNA changes to occur, creating a resistance to the once-applicable drug. D) A few drug-resistant viruses were present at the start of treatment and selective pressures increased their frequency.