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: Ans. Iodine-131', 'role': 'assistant'}]