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': 'Prostate calculi are usually composed of \nSelect and explain. \nA. Calcium carbonate, B. Calcium phosphate, C. Calcium oxalate, D. Cystine', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]