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
Extra-radicular source of persistent endodontic infection is? 
Select and explain. 
A. Propionobacterium, B. E.fecalis, C. V.parvula), D. T.denticola
A: Propionobacterium
Answer: A: Propionobacterium 
Explanation: None