This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood Physiology, Subject name: Physiology
[{'content': 'Site of RBC formation in a 20 year old healthy male is: \nSelect and explain. \nA. Flat bones, B. sho bones, C. Liver, D. Yolk sac', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Erythropoiesis : 1st trimester - in mesoderm of yolk sac (Mesoblastic stage); 2nd trimester - liver & spleen (Hepatic stage); 6 months onward - bone marrow (Myeloid stage). At bih - all bones have red bone marrow, producing cells. Progressively, bone marrow turns yellow; shafts of long bones first turn yellow, followed by their ends. By 20 years - red marrow exists only in sternum, ribs, veebrae, pelvis and skull.', 'role': 'assistant'}]