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: D i.e. Cobalt - 60