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': 'Half-Life of Cobalt-60 \nSelect and explain. \nA. 30 years, B. 5.2 years, C. 74.5 days, D. 6 hours', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Cobalt-60 – 5.2 years\nCesium-137 – 30 years\nIridum-192 – 74.5days', 'role': 'assistant'}]