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
[{'content': 'What is true regarding (A-a)O2 gradient? (A = alveolus; a = aerial) \nSelect and explain. \nA. 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', 'role': 'user'}, {'content': 'Answer: D \nExplanation: .', 'role': 'assistant'}]