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 radioisotopes is commonly used as a source for external beam radiotherapy in the treatment of cancer patients \nSelect and explain. \nA. Strontium - 89, B. Radium - 226, C. Cobalt - 59, D. Cobalt - 60', 'role': 'user'}, {'content': 'Answer: D \nExplanation: D i.e. Cobalt - 60', 'role': 'assistant'}]