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
Which of the following is obsolete in modern day clinical use? 
Select and explain. 
A. Ra226, B. C060, C. Ir192, D. Cs137
A: Ra226
Answer: A: Ra226 
Explanation: A i.e. Ra226