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': 'Cobalt – 60 is \nSelect and explain. \nA. Natural radioactive source, B. Natural radioactive material, C. Artificial radioactive source, D. Artificial stable substance', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]