Breathing Room Air, a Normal PAO₂ - PaO₂ of 5

Breathing room air, a normal PAO₂ - PaO₂ of 5 to 10 mm Hg exists due to which of the following?
I.anatomical shunts in the pulmonary and cardiac circulations
II.normal limitations to oxygen diffusion in the lung
III.regional differences in pulmonary ventilation and blood flow

A) I and III
B) III and IV
C) I, II, and III
D) I and II