One Problem Researchers Experience When Attempting to Directly Link Testosterone

One problem researchers experience when attempting to directly link testosterone to aggression and sexuality is that

A) the relationships have never been studied in women.
B) testosterone seems to affect sexual desire in men but not in women.
C) it is difficult to determine the causal direction because aggression and sex can affect testosterone levels.
D) both sex and especially aggression are difficult to study in the lab.