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
[{'content': 'Site of RBC formation in 20 year old healthy male is: \nSelect and explain. \nA. Flat bones, B. Long bones, C. Liver, D. Yolk sac', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]