This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
[{'content': "In computed tomography (CT) the attenuation value are measured in hounsefield units (HU). An attenuation value of '0' (zero) HV corresponds to - \nSelect and explain. \nA. Water, B. Very dense bone structure, C. AIR, D. Fat", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Water is arbitrarily assigned a value of zero (0) Hounsfield units (HU).', 'role': 'assistant'}]