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': 'Pinhole is used to neutralize refractive error till \nSelect and explain. \nA. 2D, B. 4D, C. 6D, D. 3D', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]