What Allows for the Transfer of Oxygen from the Bloodstream

What allows for the transfer of oxygen from the bloodstream of a mother to the bloodstream of fetus?

A) There is a partial pressure difference between the O₂ of the mother and the O₂ of the fetus.
B) The blood vessels of the fetus are larger than that of the mother.
C) The hemoglobin of the fetus is different than adults and has a greater affinity for oxygen then the hemoglobin of the mother.
D) The hemoglobin of the fetus is larger than the hemoglobin of the mother.
E) The fetus uses myoglobin to carry the oxygen in the blood,the mother uses hemoglobin.