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: Radiology
[{'content': 'Normal air fluid level in a x-ray abdomen is \nSelect and explain. \nA. 1, B. 2, C. 3, D. 4', 'role': 'user'}, {'content': 'Answer: D \nExplanation: (Ref: Grainger and Allison&;s diagnostic radiology 6th edition, chapter 25)', 'role': 'assistant'}]