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: Microbiology
Cold sterilization refers to the process of sterilization by use of: 
Select and explain. 
A. Ultraviolet rays, B. Ultrasonic vibrations, C. Infrared rays, D. Gamma rays
D: Gamma rays
Answer: D: Gamma rays 
Explanation: Since there is no detectable increase of temperature in this method, this method is commonly referred to as “cold sterilization”.
Ionizing radiations include (a) X-rays, (b) Gamma rays, and (c) Cosmic rays. 

Ionizing radiation is an excellent sterilizing agent with very high penetrating power.
These radiations penetrate deep into objects and destroy bacterial endospores and vegetative cells, both prokaryotic and eukaryotic. These are, however, not that effective against viruses.
Gamma radiation from a cobalt-60 source is used for sterilization of antibiotics, hormones, sutures, catheters, animal feeds, metal foils, and plastic disposables, such as syringes. This has also been used to sterilize and “pasteurize” meat and other food items.

Parija SC. Textbook of Microbiology & Immunology. Elsevier Health Sciences; 2014. Page:28