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
The maximum head diameter of ISO Gatesglidden drill # 1 is equivalent to: 
Select and explain. 
A. No 50 file at Do, B. No 40 file at Do, C. No 60 file at Do, D. No 80 file at Do
A: No 50 file at Do
Answer: A: No 50 file at Do 
Explanation: None