What Accounts for the Change in PO₂ Between the Atmosphere

What accounts for the change in PO₂ between the atmosphere and alveoli?
I. Water vapor pressure in alveolar gas
II. PCO₂ is higher in alveolus than in the atmospheric gas
III. Higher percentage of N₂ in alveolar gas
IV. P_B is lower in the alveoli than in the atmosphere

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