You are presented with the question "What are used for breathing air by land animals?" and the following answer choices: 
- Frogs
 - pigeons
 - gills
 - alveoli
 - fish
 - ferns
 - four limbs
 - water

Now knowing that lungs are used for breathing air by land animals and inside of the lungs are little sacks called alveoli, choose the best answer.


 

    alveoli     