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': 'National family health survey is done: \nSelect and explain. \nA. Every 5 months, B. Every 5 years, C. Every 15 months, D. Biannually', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Every 5 years', 'role': 'assistant'}]