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': 'Longest half life is seen in \nSelect and explain. \nA. Radon, B. Radium, C. Uranium, D. Cobalt', 'role': 'user'}, {'content': 'Answer: B \nExplanation: B i.e. Radium', 'role': 'assistant'}]