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': 'Commonest type of gall stone is - \nSelect and explain. \nA. Cholesterol stone, B. Pigment, C. Mixed, D. All are equally common', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]