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
Which one of the following radioisotope is not used as a permanent implant : 
Select and explain. 
A. Iodine–125, B. Palladium–103, C. Gold–198, D. Caesium–137
D: Caesium–137
Answer: D: Caesium–137 
Explanation: Caesium - 131 → Permanent implant
Caesium - 137 → Temporary implant