<prompt> Which among the following has least HU units?Acute BleedContrast agentsFatAir  <response>: Air has the least HU Coical Bone 400 to 1000 Calcifications 200 to 250 Contrast agents 130 Hu Acute Bleed 60 to 70 Hu CSFWater 0 Fat - 50 to -100 Hu AIR -1000 Hu 