This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Physiology
What is true regarding (A-a)O2 gradient? (A = alveolus; a = aerial) 
Select and explain. 
A. Normal gradient ~ 15 mmhg (2 kpa), B. An increased gradient may be caused by, pulmonary collapse, C. Venous admixture affects (A-a)O2 gradient, D. All
D: All
Answer: D: All 
Explanation: .