This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: CT SCAN, Subject name: Radiology
[{'content': 'Which among the following has least HU units? \nSelect and explain. \nA. Acute Bleed, B. Contrast agents, C. Fat, D. Air', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Air has the least HU Coical Bone +400 to +1000 Calcifications +200 to +250 Contrast agents +130 Hu Acute Bleed +60 to +70 Hu CSF/Water 0 Fat - 50 to -100 Hu AIR -1000 Hu', 'role': 'assistant'}]