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
[{'content': 'What size of donor cornea is taken in keratoplasty? \nSelect and explain. \nA. 2 mm, B. 4 mm, C. 6 mm, D. 8 mm', 'role': 'user'}, {'content': 'Answer: D \nExplanation: An ideal size is 7.5 mm. Smaller sizes would give rise to astigmatism due to subsequent tissue tension.', 'role': 'assistant'}]