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
One of the following radioisotopes is used for interstitial therapy: 
Select and explain. 
A. Phosphorus 32, B. Iodine 131, C. Iridium 191, D. Gold 198
C: Iridium 191
Answer: C: Iridium 191 
Explanation: Ans. Iridium 191