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 mandibular first molar is hemisected with separate crowns placed on the mesial and distal roots. Both roots have been treated endodontically with posts, it's long term prognosis is questionable and may be lost due to: 
Select and explain. 
A. Root fracture., B. Root caries., C. Recurrent endodontic failure., D. All of the above.
D: All of the above.
Answer: D: All of the above. 
Explanation: None