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: Radiology
In ultrasound machines, the principle is: 
Select and explain. 
A. Piezo electric crysial excitation, B. Very low frequency radiowaves, C. Microwaves, D. None of the above
A: Piezo electric crysial excitation
Answer: A: Piezo electric crysial excitation 
Explanation: Ans. Piezo electric crysial excitation