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: Radiology
[{'content': 'Investigation of choice for gall stone – \nSelect and explain. \nA. X–ray, B. USG, C. Cholecystography, D. CAT Scan', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]