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
[{'content': 'Investigation of choice in gall bladder stone is - \nSelect and explain. \nA. USG, B. X-ray abdomen, C. OCG, D. Intravenous cholangiogram', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]