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
Instrument No. 40 represents 
Select and explain. 
A. Diameter at D1, B. Diameter at D2, C. Diameter at D1+D2, D. Diameter at D2-D1
A: Diameter at D1
Answer: A: Diameter at D1 
Explanation: None