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 is Pan leukocyte marker 
Select and explain. 
A. CD19, B. CD3, C. CD45, D. CD45 RO
C: CD45
Answer: C: CD45 
Explanation: CD 45 is Pan leukocyte marker.
CD 45 RO is marker for MEMORY cell.