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
4th generation apex locators are based on? 
Select and explain. 
A. Frequency based, B. Impedence based, C. Multiple frequency ratio based, D. Resistance based
C: Multiple frequency ratio based
Answer: C: Multiple frequency ratio based 
Explanation: None