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: B i.e. Iodine -131', 'role': 'assistant'}]