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
According to instrument formula given by G.V. Black, primary cutting edge angle should be: 
Select and explain. 
A. More than 50 degree, B. More than 90 degree, C. Less than 50 degree, D. Less than 90 degree
A: More than 50 degree
Answer: A: More than 50 degree 
Explanation: The second number of a four-number code indicates the primary cutting edge angle, measured from a line parallel to the long axis of the instrument handle in clockwise centigrade.
The centigrade angle is expressed as a percent of 360 degrees (e.g., 85 = 85% × 360 degrees = 306 degrees). The instrument is positioned so that this number always exceeds 50.

Sturdevants operative dentistry 7th edition page e2