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: Pathology
Pre-B cells and B cells are produced in ? 
Select and explain. 
A. Bone marrow, B. Thymus, C. Tonsils, D. Gut associated lymphoid tissue
A: Bone marrow
Answer: A: Bone marrow 
Explanation: Ans. is 'a' i.e., Bone marrow