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': 'Breast cancer is epidemiologically commoner with ? \nSelect and explain. \nA. Smoking, B. Nulliparity, C. Oral contraceptives, D. Multiparity', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Answer (b) Nulliparity', 'role': 'assistant'}]