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': 'One of the following radioisotopes is used for interstitial therapy: \nSelect and explain. \nA. Phosphorus 32, B. Iodine 131, C. Iridium 191, D. Gold 198', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. Iridium 191', 'role': 'assistant'}]