One of the Issues with Studies Conducted on the Prevalence

One of the issues with studies conducted on the prevalence of sexual behaviors is that

A) the data are gathered from self-reports of a certain group of people and may be biased and ambiguous.
B) the data are seldom gathered anonymously.
C) the data-gathering methods do not ask about specific behaviors but classes of behaviors.
D) people responding to the questionnaires do not usually know the actual definitions of the behaviors being asked about.