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
Normal air fluid level in a x-ray abdomen is 
Select and explain. 
A. 1, B. 2, C. 3, D. 4
D: 4
Answer: D: 4 
Explanation: (Ref: Grainger and Allison&;s diagnostic radiology 6th edition, chapter 25)