Maternal-fetal blood incompatibility may exist in which of the following conditions?
a. The mother is Rh-positive and the fetus is Rh-negative.
b. The mother is Rh-negative and the fetus is Rh-positive.
c. The mother has type A blood and the fetus has type O blood.
d. The mother has type AB blood and the fetus has type B blood.
Answer: B