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: Microbiology
Which one of the following is a pan T lymphocyte marker - 
Select and explain. 
A. CD2, B. CD3, C. CD19, D. CD25
B: CD3
Answer: B: CD3 
Explanation: The most clearcut differentiation between T and B cells is by their surface markers, for example, by demonstration of CD3 on T cells and Ig on B cells.