What Action Would Be the Best Control for a Mediating

What action would be the best control for a mediating variable when designing a study to determine whether a videotape format or a pamphlet is more effective in teaching young women how to perform breast self-examination (BSE) ?

A) Randomly assign subjects to the two groups.
B) Exclude subjects who already perform BSE on a regular basis.
C) Limit the study subjects to women who are heterosexual.
D) Determine what breast cancer means to the individual participants.