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
What is torsional limit? 
Select and explain. 
A. Amount of apical pressure that can be applied to a file to the point of breakage, B. The beginning of plastic deformation of the instrument, C. Amount of rotational torque that can be applied to a “locked” instrument to the point of breakage, D. Amount of force necessary so that a file does not return to its original shape upon unloading of the force
C: Amount of rotational torque that can be applied to a “locked” instrument to the point of breakage
Answer: C: Amount of rotational torque that can be applied to a “locked” instrument to the point of breakage 
Explanation: Torsional limit is the amount of rotational torque that can be applied to a "locked" instrument to the point of breakage (separation). 
Obviously, an instrument should have sufficient strength to be rotated and worked vigorously without separating in the canal.
Smaller instruments (less than size 20) can withstand more rotations without breaking than larger (greater than size 40) instruments.

Ref :Principles and Practice of Endodontics, Richard E Walton, 3rd edition, pg no:156