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 do not contain - \nSelect and explain. \nA. Oxalate, B. Cholestrol, C. Phosphate, D. Carbonate', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e. Oxalate", 'role': 'assistant'}]