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
Fourth generation of apex locator are based on which principle 
Select and explain. 
A. Resistance based, B. Impedance based, C. Impedance ratio based, D. Multiple frequencies based
D: Multiple frequencies based
Answer: D: Multiple frequencies based 
Explanation: First generation …resistance based
Second…impedence
Third…impedence ratio
Fourth ..multifrequency