This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Pituitary-Hypothalmic System and Thyroid, Subject name: Pharmacology
The physical half life of radioactive I131 is: 
Select and explain. 
A. 8 hrs, B. 8 days, C. 16 days, D. 60 days
B: 8 days
Answer: B: 8 days 
Explanation: Half life of radioactive I131 is 8 days. It can cause irreversible hypothyroidism. It is contra-indicated in pregnancy.