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
Problems encountered during instrumentation : 
Select and explain. 
A. Ledging., B. Stripping / lateral wall Perforation ., C. Zipping., D. All of the above.
D: All of the above.
Answer: D: All of the above. 
Explanation: Problems Encountered during Instrumentation

Loss of working length due to a blockage of the canal system
Ledging (shelf formation)
Breakage of instrument
Deviation from the normal canal anatomy
Zipping (elliptication)
Stripping/lateral wall perforation
Inadequate canal preparation
Overinstrumentation (beyond apical constriction)
Over preparation (excessive removal of tooth structure)
Under preparation (insufficient canal preparation)