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
Which of the following is a secondary lymphoid organ? 
Select and explain. 
A. Liver, B. Spleen, C. Bone marrow, D. Thymus
B: Spleen
Answer: B: Spleen 
Explanation: 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