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': 'The focal length of the mirror used in head lamp - \nSelect and explain. \nA. 85mm, B. 150 mm, C. 250mm, D. 400 mm', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Head mirror is a concave mirror used to reflect light from the bull’s eye lamp onto the part being examined\nIt has focal length of ~ 25 cm (= 10 inch) The examiner sees through the hole in the centre of the mirror\nDiameter of central hole = 19 mm (3/4 inch)', 'role': 'assistant'}]