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: Gynaecology & Obstetrics
[{'content': 'Safe period is : \nSelect and explain. \nA. 1 -- 5 days, B. 8 -- 11 days, C. 16 -- 20 days, D. 21 -- 28 days', 'role': 'user'}, {'content': 'Answer: D \nExplanation: 21 -- 28 days', 'role': 'assistant'}]