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
What is the mechanism of an acoustic shadow seen on USG? 
Select and explain. 
A. Refraction, B. Reflection, C. Aifacts, D. Obstruction
B: Reflection
Answer: B: Reflection 
Explanation: An acoustic shadow is an echo free area located behind an insonated structure. It is caused by total reflection of the ultra sound waves causing a shadow behind it. Reference: The Practice of Ultrasound: A Step-by-Step Guide to Abdominal Scanning By Behold Block page 49.