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': '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: D i.e. Attenuation coefficient', 'role': 'assistant'}]