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': 'Distal Acoustic shadow is caused by which of the following? \nSelect and explain. \nA. Air, B. Calculi, C. Bone, D. All of the above', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Distal Acoustic shadow is caused by substances such as bone, stones and air which have an acoustic impedence very different from that of adjacent tissue. Because of the large impedence mismatch, much of the ultrasound energy is reflected back toward the transducer. If the reflector is insonated at an approximate right angle, it produces a very bright echo.', 'role': 'assistant'}]