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: Surgery
Investigation of choice in gall bladder stone is - 
Select and explain. 
A. USG, B. X-ray abdomen, C. OCG, D. Intravenous cholangiogram
A: USG
Answer: A: USG 
Explanation: None