This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Diagnosis of Pregnancy, Subject name: Gynaecology & Obstetrics
In "obsetrics ultra sound" commonly employed range of frequency is:- 
Select and explain. 
A. 3.5-5 MHz (Megahez), B. 5 - 7.5 MHz, C. 1 - 2.5 MHz, D. 8 - 10 MHz
A: 3.5-5 MHz (Megahez)
Answer: A: 3.5-5 MHz (Megahez) 
Explanation: Commonly employed range of frequency in obsetrics ultra sound - 3.5-5 Mega Hz ( Trans abdominal scan) Obstetrical USG Range of Frequency Trans abdominal scan (TAS) using convex probe 3.5 - 5 Mega Hz Trnsvaginal scan (TVS) using TVS probe 5 - 6.5 Mega Hz