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: Gynaecology & Obstetrics
Safe period is : 
Select and explain. 
A. 1 -- 5 days, B. 8 -- 11 days, C. 16 -- 20 days, D. 21 -- 28 days
D: 21 -- 28 days
Answer: D: 21 -- 28 days 
Explanation: 21 -- 28 days