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': 'The longest half life is that of: \nSelect and explain. \nA. Radon, B. Radium, C. Uranium, D. Cesium', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. Uranium', 'role': 'assistant'}]