During erythroblastosis fetalis, a Rh- mother's anti-Rh antibodies that have crossed the placenta will cause agglutination of the fetus's Rh+ RBCs. However, the reverse problem never happens when a Rh+ mother is pregnant with a Rh- fetus, that is, antibodies produced by the fetus cannot cause agglutination of the mother's Rh+ RBCs. This is true because ______.
-agglutinins are physically too large to pass across the placenta
-fetal antibodies are immature and non-functional
-antibodies that can cause this agglutination are not produced by a fetus
-the placenta is a barrier that prevents the passage of all antigens