This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Elementary Optics, Subject name: Ophthalmology
[{'content': 'Placido disc is used for measuring? \nSelect and explain. \nA. Corneal thickness, B. Corneal surface, C. Corneal staining, D. Corneal curvature', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. (b) Corneal surface', 'role': 'assistant'}]