Why Do Harmful Agents Cause the Most Severe Birth Defects

Why do harmful agents cause the most severe birth defects during the embryonic period?

A) This is a period of rapid differentiation when cells and tissues acquire specific functions, and organs form.
B) This is when substances pass most easily through the placenta.
C) The mother's hormones are at the highest levels at this time.
D) The embryo has not yet implanted into the uterus.