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': 'Cataract is a cause of: \nSelect and explain. \nA. Painful sudden vision loss, B. Painless sudden vision loss, C. Painful gradual vision loss, D. Painless gradual vision loss', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Painless gradual vision loss', 'role': 'assistant'}]