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: Unknown
The commonly used frequency range in obstetrical ultrasonography is - 
Select and explain. 
A. 3.5-5 KHz ', B. 20-25 KHz, C. 3.5-5 MHz, D. 20-25 MHz
C: 3.5-5 MHz
Answer: C: 3.5-5 MHz 
Explanation: Ans: C