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 success of a root canal filling is best assessed \nSelect and explain. \nA. Clinical observation, B. Size of gutta-percha cone used, C. Radiographs, D. By the diameter of root canal reamer', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]