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': 'Focal length of head mirror used in ENT-OPD - \nSelect and explain. \nA. 9 inch, B. 10 inch (25 cm), C. 11 inch, D. 12 inch', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]