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
Which among the following has least HU units? 
Select and explain. 
A. Acute Bleed, B. Contrast agents, C. Fat, D. Air
D: Air
Answer: D: Air 
Explanation: 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