Which of the Following Is Not an Explanation for the Difficulty

Which of the following is not an explanation for the difficulty in determining the teratogenic effects of illicit drug use?

A) Women who abuse drugs rarely maintain the use of a single drug during pregnancy.
B) Poor nutrition and lower socioeconomic status often accompany drug use.
C) The incidence of drug abuse during pregnancy has hit an unprecedented low,making research in this area of study difficult to conduct.
D) Drug-abusing mothers are less likely to receive proper prenatal care.