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': 'All are Bcell markers except \nSelect and explain. \nA. CD 15, B. CD 19, C. CD 21, D. CD 23', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]