DrJones Wants to Determine Whether Women Who Smoke While Pregnant

Dr.Jones wants to determine whether women who smoke while pregnant will have children with more respiratory problems than those born to mothers who do not smoke.The reason Dr.Jones cannot conduct an experiment is because

A) women in the control group might still have children with respiratory problems.
B) the placebo group needs to be given cigarettes as well.
C) women in the control group would be manipulated.
D) assignment of women to the experimental group would be unethical.