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
Diameter of head mirror 
Select and explain. 
A. 3 inches, B. 3.5 inches, C. 2.5 inches, D. 9 mm
B: 3.5 inches
Answer: B: 3.5 inches 
Explanation: None