This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Ear, Subject name: ENT
Diameter of head mirror in ENT is 
Select and explain. 
A. 20 cm, B. 22 cm, C. 10 cm, D. 26 cm
C: 10 cm
Answer: C: 10 cm 
Explanation: Head mirror: It is a concave mirror used to reflect light from the Bull's eye lamp onto the pa being examined. It has a focal length of approximately 25 cm. The examiner sees through the hole in the centre of the mirror. Diameter of the mirror is 89 mm (3 1/2 ") and that of the central hole is 19 mm (3/4") Ref: Dhingra 7e pg 427.