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': 'A gall stone gets impacted most commonly in which pa of common bile duct - \nSelect and explain. \nA. Supraduodenal, B. Retroduodenal, C. Ampulla of vater, D. Common hepatic duct', 'role': 'user'}, {'content': "Answer: C \nExplanation: Ans. is 'c' i.e. Ampulla of vater", 'role': 'assistant'}]