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: Kim et al classification of isthmus
Classification of the isthmus : by Hus & Kim et al
Type I - Is two or three canals with no notable communication.
Type II - Is two canals that possess a definite connection between the two main canals.
Type III - Is three canals that possess a definite connection between them.
Type IV- Is when the canals extend into the isthmus area.
Type V- Is the true connection or corridor throughout the section.