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: Cobalt - 60 and Cesium - 137 are most commonly used radioisotopes for teletherapy (External beam radiation).', 'role': 'assistant'}]