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
Type-IV isthmii is? 
Select and explain. 
A. complete or incomplete isthmus between three or more canals, B. Complete communication between two canal, C. No communication between any canal, D. two or three canal openings without visible connections
A: complete or incomplete isthmus between three or more canals
Answer: A: complete or incomplete isthmus between three or more canals 
Explanation: None