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': 'Frequency for trans vaginal USG for obstetric purpose \nSelect and explain. \nA. 3 - 5 MHz, B. 5 -7.5  MHz, C. 7.5 - 20  MHz, D. > 20 MHz', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Transabdominal USG -\xa03 - 5\xa0MHz.\nTrans vaginal USG - 5\xa0- 7.5\xa0MHz.', 'role': 'assistant'}]