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: Dental
[{'content': 'The recommended rubber dam retainer for maxillary molar anchor teeth is \nSelect and explain. \nA. W7, B. W8, C. W4, D. W2', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]