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
Which of the following type of cells give rise to macrophages? 
Select and explain. 
A. Neutrophils, B. Monocytes, C. Lymphocytes, D. Reticulo endothelial cells.
B: Monocytes
Answer: B: Monocytes 
Explanation: Monocytes formed in the bone marrow are precursors of tissue macrophages. After an intravascular stay of 3 days, monocytes migrate from the blood into tissues. In the tissues, monocytes undergo differentiation into macrophages.