This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nuclear Medicine, Subject name: Radiology
[{'content': 'Radioisotope used in PET scan is? \nSelect and explain. \nA. Tachnetium 99m, B. Iodine 123, C. Iodine 131, D. Fluoride-18', 'role': 'user'}, {'content': "Answer: D \nExplanation: Ans. (d) Fluoride-18Ref. Sutton's radiology, 7th ed. H826", 'role': 'assistant'}]