One of the Reasons Scientists Have Had So Much Difficulty

One of the reasons scientists have had so much difficulty finding a cure for HIV/AIDS is that the HIV virus is quick to evolve resistance to the drugs used to treat it. What is the most likely mechanism for this type of evolution?

A) Certain versions of the virus are slower to die in the presence of a drug than others. Those viruses may survive drug treatment and pass their genes to their offspring.
B) Every time an HIV drug it taken, it randomly kills off most but not all virus particles. Those that survive repopulate the body of their host.
C) Although all copies of a particular strain of the HIV virus are identical, some are better at "hiding" in regions of the body that drugs cannot reach, and those viruses "teach" their offspring where to hide.
D) Some versions of the HIV virus produce chemicals that break down the drugs before they can harm the virus.