This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fundamentals in Radiology, Subject name: Radiology
[{'content': 'Hounsfield unit is zero for which of the following? \nSelect and explain. \nA. Air, B. Water, C. Fat, D. Dense bone', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Hounsfield units are used to measure the X ray attenuation in CT scan. Depends on absorption/attenuation coefficient. Water is assigned a HU value of 0.', 'role': 'assistant'}]