You are presented with the question "Thirst shows an animal that it needs what?" and the following answer choices: 
- CO2
 - H2O
 - He
 - food
 - feed
 - urea
 - O2
 - H20

Now knowing that thirst is used to make an animal realize that it needs to replenish its bodily water by the body and an example is H2O which shows water is made of two atoms of hydrogen stuck to one atom of oxygen, choose the best answer.


 

  H2O       