Low Partial Pressure of Oxygen in the Atmosphere Has What

Low partial pressure of oxygen in the atmosphere has what effect on the oxygen diffusion into the blood from alveoli?

A) The diffusion rate of oxygen into the blood is always constant and does not change due to a change in the number of available oxygen molecules in the alveolar air.
B) The diffusion rate is increased because there are more oxygen molecules available in the alveolar air.
C) The diffusion rate is decreased because there are more oxygen molecules available in the alveolar air.
D) The diffusion rate is increased because there are fewer oxygen molecules available in the alveolar air.
E) The diffusion rate is decreased because there are fewer oxygen molecules available in the alveolar air.