This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General Embryology 1, Subject name: Anatomy
Teiary chorionic villi develop at? 
Select and explain. 
A. Day 28, B. Day 25, C. Day 17, D. Day 15
C: Day 17
Answer: C: Day 17 
Explanation: Teiary chorionic villi develop at day 17-21