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': 'For teletherapy, isotopes commonly used are \nSelect and explain. \nA. 1-123, B. Cs-137, C. Co-60, D. Tc-99', 'role': 'user'}, {'content': 'Answer: C \nExplanation: C i.e. Co-60', 'role': 'assistant'}]