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
National family health survey is done: 
Select and explain. 
A. Every 5 months, B. Every 5 years, C. Every 15 months, D. Biannually
B: Every 5 years
Answer: B: Every 5 years 
Explanation: Every 5 years