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
[{'content': 'Lancefield classification based on: \nSelect and explain. \nA. M protein, B. Carbohydrate antigen, C. Cell wall peptidogglycan, D. T protein', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Carbohydrate antigen', 'role': 'assistant'}]