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': 'Which one of the following radioisotope is not used as permanent implant? \nSelect and explain. \nA. Iodine-I25, B. Palladium-103, C. Gold-198, D. Caesium-137', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Ans. Caesium-137', 'role': 'assistant'}]