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 permanent implant? 
Select and explain. 
A. Iodine-I25, B. Palladium-103, C. Gold-198, D. Caesium-137
D: Caesium-137
Answer: D: Caesium-137 
Explanation: Ans. Caesium-137