The Placenta Is Fed by Blood Vessels from the Mother

The placenta is fed by blood vessels from the mother and the embryo/fetus.How are the two blood supplies related?

A) The two blood systems are interconnected so that the mother's blood flows directly into the placental villi.
B) The vessel walls serve as semipermeable membranes, but the two bloodstreams do not mix.
C) The placenta carries embryo/fetal waste products out but allows nothing to enter from the mother's bloodstream.
D) The placenta acts as a semipermeable barrier that allows only oxygen and nutrients to pass to the embryo/fetus.