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': 'Which is the smallest part of optic nerve \nSelect and explain. \nA. Intraocular, B. Intraorbital, C. Intracanalicular, D. Intracranial', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Intraocular - 1 mm.\nIntraorbital - 30 mm.\nIntracanalicular - 6-9 mm.\nIntracranial - 10 mm.', 'role': 'assistant'}]