If a Person Is Treated with a Single Anti-HIV Drug,they

If a person is treated with a single anti-HIV drug,they often end up with a resistant strain.How does this occur?

A) The drugs kill all sensitive viruses, leaving only the preexisting resistant viruses.
B) The HIV wants to avoid the drug, so it becomes resistant.
C) The drug causes mutations in the HIV, making them resistant to the drug.
D) The drugs weaken the immune system, making a patient more susceptible to infection with resistant strains of HIV.