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': "Snellen's cha is used to test: \nSelect and explain. \nA. Vision, B. Refraction, C. Presbyopia, D. Colour blindness", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. Vision', 'role': 'assistant'}]