Why Do Rh-Negative but Not Rh-Positive Mothers Sometimes Have Babies

Why do Rh-negative but not Rh-positive mothers sometimes have babies with hemolytic disease of the newborn?

A) This disease results when an Rh-negative mother's immune system is primed multiple times to produce anti-Rh IgG antibodies that can cross the placenta.If the mother is Rh-positive,she won't produce any anti-Rh antibodies at all.
B) Rh-positive mothers produce IgM antibody,not IgG.This antibody can't cross the placenta,so it can't cause hemolytic disease of the newborn.
C) Rh-positive mothers will receive a preventative shot from their physician prior to conception.This will provide the protection the fetus needs AFTER conception to avoid the disease.
D) Rh-negativity is also associated with hyperproduction of antibodies.As such,Rh-negative mothers are more likely than Rh-positive mothers to produce the antibodies needed to produce this disease.