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
[{'content': 'In an aphakic eye posterior focal point from the back of cornea is about: \nSelect and explain. \nA. 23 mm, B. 25 mm, C. 31 mm, D. 21 mm', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. 31 mm', 'role': 'assistant'}]