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
While sharpening curettes and sickles, the internal angle
of _________ must be preserved between the face and lateral it
surface 
Select and explain. 
A. 50-60, B. 60-70, C. 70-80, D. 80-90
C: 70-80
Answer: C: 70-80 
Explanation: None