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
Cobalt – 60 is 
Select and explain. 
A. Natural radioactive source, B. Natural radioactive material, C. Artificial radioactive source, D. Artificial stable substance
C: Artificial radioactive source
Answer: C: Artificial radioactive source 
Explanation: None