Erythroblastosis foetalis occurs when

Question

$\begin{array}{1 1}(A)\;\text{Father is }Rh^+\text{ and mother is }Rh^-\\(B)\;\text{Father is }Rh^-\text{ and mother is }Rh^+\\(C)\;\text{Both are }Rh^+\\(D)\;\text{Both are }Rh^-\end{array}$