One of the Reasons Why Scientists Have Had So Much

One of the reasons why scientists have had so much difficulty finding a cure for HIV (human immunodeficiency virus) and AIDS is that the virus is quick to evolve resistance to the drugs that are 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 when exposed to a drug than others. Those viruses may survive drug treatment and pass their genes to their offspring.
B) Every time a person takes an HIV drug, it randomly kills off most of the virus particles but not all of them. Those that survive repopulate the body of their host.
C) Although all copies of a particular strain of HIV 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 HIV produce chemicals that break down the drugs before they can harm the virus.