This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Physiology
What size of donor cornea is taken in keratoplasty? 
Select and explain. 
A. 2 mm, B. 4 mm, C. 6 mm, D. 8 mm
D: 8 mm
Answer: D: 8 mm 
Explanation: An ideal size is 7.5 mm. Smaller sizes would give rise to astigmatism due to subsequent tissue tension.