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
Registration of bihs should be done 
Select and explain. 
A. After 21 days, B. Before 21 days, C. Within 15days, D. Within 30 days
B: Before 21 days
Answer: B: Before 21 days 
Explanation: Ref:Park's Textbook of Preventive and Social Medicine 25th Ed