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[{'content': 'Which of the following is obsolete in modern day clinical use? \nSelect and explain. \nA. Ra226, B. C060, C. Ir192, D. Cs137', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. Ra226', 'role': 'assistant'}]