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': 'While sharpening curettes and sickles, the internal angle\nof _________ must be preserved between the face and lateral it\nsurface \nSelect and explain. \nA. 50-60, B. 60-70, C. 70-80, D. 80-90', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]