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': 'Piezoelectric crystals are made use of that is safe from radiation also: \nSelect and explain. \nA. MRI, B. US, C. CT, D. All', 'role': 'user'}, {'content': 'Answer: B \nExplanation: B i.e. US Type of USG USG frequency - Trans abdominal USG for 3-5 MHzQ obstetric purpose 5- 7.5 MHzQ - Trans vaginal USG for obstetric purpose - USG for breast 15 MHz - Endoscopic USG for gutwall 7.5 - 20 MHz - To image vessel wall 20 MHz - cathater', 'role': 'assistant'}]