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': 'Isotope which is replacing radium is: \nSelect and explain. \nA. Cesium, B. Iridium, C. Gold, D. Californium', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. Iridium', 'role': 'assistant'}]