Why is it difficult to determine the effects of prenatal exposure to cocaine and heroin?

A) Most of these babies cannot be identified by health-care workers.
B) Many of these babies were exposed in the first trimester, making it difficult to determine its effects on older fetuses.
C) Most of these babies were exposed to other substances, making it difficult to isolate the effect of each factor.
D) Drug-abusing mothers often refuse to have their babies tested for illegal drugs.