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: ENT
[{'content': 'Diameter of head mirror \nSelect and explain. \nA. 3 inches, B. 3.5 inches, C. 2.5 inches, D. 9 mm', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]