What Stops Oxygen from Diffusing Back into the Lungs After

What stops oxygen from diffusing back into the lungs after it is in the blood?

A) Diffusion slows once there are equal concentrations in the blood and lungs.
B) The oxygen levels in the alveoli and blood move toward equilibrium.
C) The blood is flowing past the alveoli in capillaries and out to the heart.
D) Oxygen does diffuse back into the lungs in small amounts,but the overall movement of oxygen is from the high concentration in the lungs to the low concentration in the blood.
E) All of the answers are correct.