This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunity, Subject name: Pathology
[{'content': 'Which is a flow cytometric B cell marker? \nSelect and explain. \nA. CD 2, B. CD 3, C. CD 7, D. CD 19', 'role': 'user'}, {'content': 'Answer: D \nExplanation: * B cell markers are CD 19, 20, 21 and 23.* TH cell markers are CD 4, 3, 7, 2* TC cell markers are CD 8, 3, 7, 2* NK cell markers are CD 2, 16, 56', 'role': 'assistant'}]