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
When instrument with primary cutting edge is perpendicular to blade then: 
Select and explain. 
A. 1st digit of 4 digit formula omitted., B. 2nd digit of 4 digit formula omitted., C. 3rd digit of 4 digit formula omitted., D. 4th digit of 4 digit formula omitted.
B: 2nd digit of 4 digit formula omitted.
Answer: B: 2nd digit of 4 digit formula omitted. 
Explanation: None