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
[{'content': 'Which is not echogenic while doing ultrasonography- \nSelect and explain. \nA. Bile, B. Gas, C. Bone, D. Gall stones', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Bile and urine are least echogenic.\nBone, gas, and organ boundaries are most echogenic.', 'role': 'assistant'}]