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
The primary gutta-percha cone must fill the canal wall tightly in the: 
Select and explain. 
A. apical third, B. middle third, C. cervical third, D. entire canal
A: apical third
Answer: A: apical third 
Explanation: None