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
The foetal circulation is first intact and functional, separated from maternal circulation at the age of: 
Select and explain. 
A. 8 days, B. 12 days, C. 17 days, D. 21 days
D: 21 days
Answer: D: 21 days 
Explanation: None