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': 'Central lymphoid organs are: \nSelect and explain. \nA. Bone marrow, B. Lymph node, C. Spleen, D. All', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. Bone marrow', 'role': 'assistant'}]