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': 'What is the length of optic nerve \nSelect and explain. \nA. 20 mm, B. 30 mm, C. 40 mm, D. 50 mm', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Answer- D. 50 mmoptic nerve is about 47-50 mm in length and can be divided into 4 pas : -Intraocular pa (1 mm)Intraorbital pa (30 mm)Intracanalicular pa (5-9 mm)Intracranial pa (10 mm)', 'role': 'assistant'}]