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': 'Radioisotopes which is used in teletherapy: September 2011 \nSelect and explain. \nA. Ra-226, B. Cs-137, C. Co-60, D. 1r-192', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. Co-60', 'role': 'assistant'}]