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: Physiology
Site of RBC formation in 20 year old healthy male is: 
Select and explain. 
A. Flat bones, B. Long bones, C. Liver, D. Yolk sac
A: Flat bones
Answer: A: Flat bones 
Explanation: None