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: Ans. Cobalt-60