This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Biostatistics, Subject name: Social & Preventive Medicine
[{'content': 'Registration of bihs should be done \nSelect and explain. \nA. After 21 days, B. Before 21 days, C. Within 15days, D. Within 30 days', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ref:Park's Textbook of Preventive and Social Medicine 25th Ed", 'role': 'assistant'}]