One of the Problems with Determining the Impact of Stress

Multiple Choice

One of the problems with determining the impact of stress on human pregnancy is that

A) studies are correlational,so cause and effect cannot be determined.
B) most pregnant women do not experience stress,so it is hard to study.
C) stress is not confounded with other variables,like smoking and drinking.
D) stress in animals manifests itself in the same way as stress in humans.