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': 'All of the following are causes of sudden painless loss of vision except: \nSelect and explain. \nA. Central retinal aery occlusion, B. Optic atrophy, C. Optic neuritis, D. Retinal detachment', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. Optic atrophy', 'role': 'assistant'}]