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': '. Gall stones - \nSelect and explain. \nA. Are about twice as common in men as in women, B. There is an increased incidence of stones in diabetics, C. About 80-90% of gall stones are radio-opaque, D. Are usually more than 50 mm in diameter', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]