This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Social & Preventive Medicine
Sample registration system is done once in ? 
Select and explain. 
A. 6 months, B. 1 year, C. 2 years, D. 5 years
A: 6 months
Answer: A: 6 months 
Explanation: Ref: Park 24th edition