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
Which is not echogenic while doing ultrasonography- 
Select and explain. 
A. Bile, B. Gas, C. Bone, D. Gall stones
A: Bile
Answer: A: Bile 
Explanation: Bile and urine are least echogenic.
Bone, gas, and organ boundaries are most echogenic.