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': 'Which of the following radioactive isotopes is not used for brachytherapy – \nSelect and explain. \nA. Iodine–125, B. Iodine–131, C. Cobalt–60, D. Iridium–192', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]