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 radioisotopes is commonly used as a source for external beam radiotherapy in the treatment of cancer patients – 
Select and explain. 
A. Strontium–89, B. Radium–226, C. Cobalt–59, D. Cobalt–60
D: Cobalt–60
Answer: D: Cobalt–60 
Explanation: Cobalt - 60 and Cesium - 137 are most commonly used radioisotopes for teletherapy (External beam radiation).