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
[{'content': 'In ultrasound machines, the principle is: \nSelect and explain. \nA. Piezo electric crysial excitation, B. Very low frequency radiowaves, C. Microwaves, D. None of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. Piezo electric crysial excitation', 'role': 'assistant'}]