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': 'Main cause of failure of endodontic therapy: \nSelect and explain. \nA. Improper biomechanical preparation, B. Improper access cavity preparation, C. Incomplete obturation, D. Over extended filing', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]