Parents Ask the Nurse, "What Makes the Opening Between the Baby's

Parents ask the nurse, "What makes the opening between the baby's atriums close at birth?" The nurse's response is that cardiovascular changes that cause the foramen ovale to close at birth are a direct result of:

A) changes in the hepatic blood flow.
B) increased pressure in the left atrium.
C) increased pressure in the right atrium.
D) decreased blood flow to the left ventricle.