A Patient Breathing 40% O₂ at Sea Level Has a PaO₂

A patient breathing 40% O₂ at sea level has a PaO₂ of 50 mm Hg, a PCO₂ of 30 mm Hg, and a PAO₂ - PaO₂ of 250 mm Hg. When the FiO₂ is raised to 0.7, the PaO₂ rises to only 58 mm Hg. Hypoxemia is primarily due to which of the following?

A) Hypoventilation
B) Impaired diffusion
C) Right-to-left shunt
D) imbalance