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
Kim et al classification system deals with: 
Select and explain. 
A. Apical constriction, B. Apical foramen, C. Isthmus, D. No. of canals
C: Isthmus
Answer: C: Isthmus 
Explanation: According to Kim et al Classification; there are 5 types

Type I incomplete isthmus between 2 canals.
Type II well defined, complete isthmus between 2 canals.
Type III very short but complete isthmus between 2 canals.
Type IV 3 or more canals with the complete or incomplete isthmus.
Type V 3 or more canals without any visible connection.