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': 'Speed of X-ray is equal to: \nSelect and explain. \nA. Speed of light, B. Speed of electrons in X-ray tube, C. Tube voltage, D. All of the above', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. Speed of light', 'role': 'assistant'}]