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
Central lymphoid organs are: 
Select and explain. 
A. Bone marrow, B. Lymph node, C. Spleen, D. All
A: Bone marrow
Answer: A: Bone marrow 
Explanation: A i.e. Bone marrow