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: Medicine
Progenitor hematopoietic stem cells originate in 
Select and explain. 
A. Bone marrow, B. Thymus, C. Lymph Node, D. Spleen
A: Bone marrow
Answer: A: Bone marrow 
Explanation: None