This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Techniques of Ocular Examination and Diagnostic Tests, Subject name: Ophthalmology
[{'content': 'The instrument shown below is: \nSelect and explain. \nA. Maddox rod, B. Retinoscope, C. Pachymeter, D. Placido disk', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Ans. (d) Placido disk.', 'role': 'assistant'}]