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': 'Density of a radiograph depends on : \nSelect and explain. \nA. Thickness of  object, B. The quality and quantity of X-rays., C. The velocity of e - emitted from the cathode., D. All of the above', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Density / Overall darkening of a radiograph depends on:\n\nThickness of\xa0object\nThe quality and quantity of X-rays.\nThe velocity of e -\xa0emitted from the cathode.', 'role': 'assistant'}]