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?
1) Anatomical shunts in the pulmonary and cardiac circulations
2) Normal limitations to O₂ diffusion in the lung
3) Regional differences in pulmonary ventilation and blood flow

A) 1 and 3 only
B) 2 and 4 only
C) 1, 2, and 3
D) 1 and 2 only