This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Basic Concepts, Subject name: Pathology
[{'content': 'Which of the following is a secondary lymphoid organ? \nSelect and explain. \nA. Liver, B. Spleen, C. Bone marrow, D. Thymus', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Generative (Central) Lymphoid Organs Thymus: Site of development of T cells Bone marrow: Site of maturation of B lymphocytes Secondary (Peripheral) Lymphoid Organs (in which adaptive immune responses to microbes are initiated) Lymph nodes Spleen Mucosal & Cutaneous lymphoid tissues', 'role': 'assistant'}]