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
Which of the following techniques use piezoelectric crystals? 
Select and explain. 
A. Ultrasonography, B. X–ray diffraction, C. NMR imaging, D. Xeroradiography
A: Ultrasonography
Answer: A: Ultrasonography 
Explanation: Ultrasonography is based on piezoelectric effect.
MRI is based on gyeromagnetic property of proton (H+).