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': 'Sample registration survey is conducted every \nSelect and explain. \nA. 6 months, B. 1 year, C. 10 years, D. 2 years', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ref:Park's Textbook of Preventive and Social Medicine 25th Ed", 'role': 'assistant'}]