The Initial Spread of AIDS Among IV Drug Users and Gay

The initial spread of AIDS among IV drug users and gay men is believed to have occurred because:

A) these are small, closed populations in which an individual is more likely to be exposed to the virus repeatedly.
B) these groups tend to engage in many other high-risk behaviors.
C) immunocompetence is already low among these groups.
D) education and medical literacy among these groups is significantly lower than among other populations.