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 bih & death with a 6 monthly survey is done in- \nSelect and explain. \nA. National sample survey, B. Vital statistical system, C. Census, D. Sample registration system', 'role': 'user'}, {'content': 'Answer: D \nExplanation: .', 'role': 'assistant'}]