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': 'All of the following are risk factors for deep vein thrombosis (DVT) except - \nSelect and explain. \nA. Duration of surgery more than thirty minutes, B. Obesity, C. Age less than forty years, D. Use of the oestrogen-progesterone contraceptive pills', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]