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: Social & Preventive Medicine
[{'content': 'Which of the following is the national level system that provides annual national as well as state level reliable estimation of fertility and mortality - \nSelect and explain. \nA. Civil registration system, B. Census, C. Adhoc survey, D. Sample registration system', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]