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
Frequency for trans vaginal USG for obstetric purpose 
Select and explain. 
A. 3 - 5 MHz, B. 5 -7.5  MHz, C. 7.5 - 20  MHz, D. > 20 MHz
B: 5 -7.5  MHz
Answer: B: 5 -7.5  MHz 
Explanation: Transabdominal USG - 3 - 5 MHz.
Trans vaginal USG - 5 - 7.5 MHz.