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
To contain gutta-percha within the canal, instrumentation should result in: 
Select and explain. 
A. Cleaning beyond the anatomic apex, B. Cleaning beyond the radiographic apex, C. A cylindrical canal shape, D. The narrowest cross-sectional diameter at the terminus
D: The narrowest cross-sectional diameter at the terminus
Answer: D: The narrowest cross-sectional diameter at the terminus 
Explanation: The instrumentation and obturation should not extend beyond the apical foramen.
Whereas the guideline of 1 mm from the radiographic apex remains rational when using radiographs, the point of apical termination of the preparation and obturation remains empirical.
The need to compact the gutta-percha and sealer against the apical dentin matrix (constriction of the canal) is necessary to prevent extrusion of materials into the periapical tissues.

Textbook of Endodontics 3rd Ed P: