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': 'USG is/are based on: \nSelect and explain. \nA. Piezoelectric effect, B. Diamagnetic effect, C. Paramagnetic effect, D. Ferromagnetic effect', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. A. Piezoelectric effectUltrasonography:Ultrasonic image(sonographic/echographic) is based on mechanical oscillations of the crystal excited by electrical pulses (Piezoelectric effect).', 'role': 'assistant'}]