This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General anatomy, Subject name: Anatomy
The term physical half life is applicable to 
Select and explain. 
A. Respiratory preparation, B. Prodrugs, C. Radioactive isotopes, D. Alkylating agents
C: Radioactive isotopes
Answer: C: Radioactive isotopes 
Explanation: Physical half-life is defined as the period of time required to reduce the radioactivity level of a source to exactly one half its original value due solely to radioactive decay. The physical half-life is designated tphys or more commonly t1/2 . Refer 6/e p252