In Sampling, Volunteer Bias in Sex Research Would Not Be

In sampling, volunteer bias in sex research would not be a problem if:

A) those who refuse to participate are identical in their sexual behavior to those who do participate.
B) a very small sample size is taken.
C) those who refuse to participate drop out immediately, allowing for others to be sampled in their place.
D) the differences in the sexual behavior of those who participated are significantly large.