It Is Difficult to Ascertain the Direct Influence of a Mother's

It is difficult to ascertain the direct influence of a mother's marijuana use on her developing fetus because:

A) marijuana users do not generally participate in studies.
B) pregnant women who use marijuana tend to be poorer, less educated, use more other illegal drugs, and receive less prenatal care than non-marijuana users.
C) marijuana is not regularly used in the United States.
D) marijuana is not generally considered a risk factor.