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': 'Zero HU units is of- \nSelect and explain. \nA. Water, B. Fat, C. Soft tissue, D. Bone', 'role': 'user'}, {'content': 'Answer: A \nExplanation: 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', 'role': 'assistant'}]