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
Placido disc is used for measuring? 
Select and explain. 
A. Corneal thickness, B. Corneal surface, C. Corneal staining, D. Corneal curvature
B: Corneal surface
Answer: B: Corneal surface 
Explanation: Ans. (b) Corneal surface