Why Is the Rh-Positive Firstborn of an Rh-Negative Mother Unaffected

Why is the Rh-positive firstborn of an Rh-negative mother unaffected by Rh hemolytic disease of the fetus and newborn (Rh HDFN) ?

A) The plasma volume of the mother is tripled during the first pregnancy which dilutes anti-D.
B) The titer of anti-D is too low to cause destruction of fetal antigens.
C) The mother has not been immunized to the D antigen before placental separation.
D) None of the above