This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
Most common cause of hypoxemia is- 
Select and explain. 
A. Lowered inspired PO2, B. Hypoventilation, C. 'Ventialtion perfusion mismatch, D. Decreased diffusing capacity
D: Decreased diffusing capacity
Answer: D: Decreased diffusing capacity 
Explanation: None