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
[{'content': 'The primary gutta-percha cone must fill the canal wall tightly in the: \nSelect and explain. \nA. apical third, B. middle third, C. cervical third, D. entire canal', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]