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
[{'content': 'The marker for B lymphocyte is - \nSelect and explain. \nA. CD 19, B. CD 68, C. CD 34, D. CD 4', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., CD 19", 'role': 'assistant'}]