It Is Difficult to Isolate the Precise Damage Caused by Illegal

It is difficult to isolate the precise damage caused by illegal drugs during pregnancy because __________.

A) most drug-using mothers refuse to participate in teratology research
B) the majority of babies born to drug-using mothers do not survive for more than a few days
C) most drug-using mothers quit using illegal drugs during the first trimester of pregnancy
D) drug users often take several drugs, display other high-risk behaviors, and suffer from other stresses