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': "Landolt's broken ring test is used for testing: \nSelect and explain. \nA. Form sense, B. Contrast sense, C. Central field, D. Scotopic vision", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. Form sense', 'role': 'assistant'}]