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: Ophthalmology
Diameter of the optic disc is: 
Select and explain. 
A. 1.5 mm, B. 2.5 mm, C. 3.5 mm, D. 5 mm
A: 1.5 mm
Answer: A: 1.5 mm 
Explanation: Ans. 1.5 mm