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: Dental
[{'content': 'Instrument No. 40 represents \nSelect and explain. \nA. Diameter at D1, B. Diameter at D2, C. Diameter at D1+D2, D. Diameter at D2-D1', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]