This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fertilization and Development of Embryo, Subject name: Anatomy
Three layers of embryo are formed at what age? 
Select and explain. 
A. 8 days, B. 12 days, C. 16 days, D. 21 days
D: 21 days
Answer: D: 21 days 
Explanation: Ans. (d) 21 daysRef: Langmanris embryology 10th ed ch:5