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: Anatomy
The primary germ layer endoderm is derived from: 
Select and explain. 
A. Embryonal disc, B. Yolk sac, C. Amniotic cavity, D. Placenta
B: Yolk sac
Answer: B: Yolk sac 
Explanation: None