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
Zero HU units is of- 
Select and explain. 
A. Water, B. Fat, C. Soft tissue, D. Bone
A: Water
Answer: A: Water 
Explanation: Hounsfield unit: CT scan is based on linear attenuation of X-ray Linear attenuation is measured in Hounsfield units Hypodense -0 or less than zero Hyperdense- More than zero NOTE: * The reference value for Hounsfield unit is 0 seen with CSF or Water * HU of (-50) - (-100) is seen with fat * HU of (-1000) is seen with air * HU of 60 - 70 Acute bleed * HU of 130 Contrast * HU of 200 to +250 Calcifications * HU of 400 to +1000 Coical bone