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
Main cause of failure of endodontic therapy: 
Select and explain. 
A. Improper biomechanical preparation, B. Improper access cavity preparation, C. Incomplete obturation, D. Over extended filing
C: Incomplete obturation
Answer: C: Incomplete obturation 
Explanation: None