What Causes Much of the Blood in the Fetal Right

What causes much of the blood in the fetal right atrium to be shunted to the left atrium through the foramen ovale?

A) The blood pressure in the right atrium is greater than in the left atrium.
B) There is a valve that directs the blood that way.
C) The fetus is in an antigravity situation provided by the amnionic fluid.
D) The blood pressure in the left atrium is greater than in the right atrium.
E) The umbilical arteries have valves.