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 values are measured in Hounsfield units (HU). An attenuation value of '0' (zero) HU corresponds to: \nSelect and explain. \nA. Water, B. Air, C. Very dense bone structures, D. Fat", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. Water', 'role': 'assistant'}]