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
Which of the following device works on the principle of electric conductance? 
Select and explain. 
A. DIAGNOdent, B. QLF, C. CarieScan Pro, D. None of the above
D: None of the above
Answer: D: None of the above 
Explanation: Devices using electric conductance are:

Vanguard electronic caries detector 
Caries meter
Modified electrochemical impedance spectroscopy
Electrical impedance tomography