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
The focal length of the mirror used in head lamp - 
Select and explain. 
A. 85mm, B. 150 mm, C. 250mm, D. 400 mm
C: 250mm
Answer: C: 250mm 
Explanation: Head mirror is a concave mirror used to reflect light from the bull’s eye lamp onto the part being examined
It has focal length of ~ 25 cm (= 10 inch) The examiner sees through the hole in the centre of the mirror
Diameter of central hole = 19 mm (3/4 inch)