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': 'Source of gamma rays is: \nSelect and explain. \nA. Radium, B. Cobalt, C. Cesium, D. Xenon', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. Cobalt', 'role': 'assistant'}]