Fact 1: Oxygen is essential for cellular respiration for all aerobic organisms.

Fact 2: Air is used as a source of oxygen.

Given the two facts above, answer the question "What is used for cellular respiration for aerobic organisms?" with the following options: 
- air
 - O2
 - O3
 - RNA
 - CO 2
 - fat
 - lung
 - tube


 

 air        