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 units depends on - \nSelect and explain. \nA. Electron density, B. Mass density, C. Effective atomic number, D. Attenuation coefficient', 'role': 'user'}, {'content': 'Answer: D \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'}]