Fact 1: Lungs are used for breathing air by land animals.

Fact 2: Inside of the lungs are little sacks called alveoli.

Given the two facts above, answer the question "What are used for breathing air by land animals?" with the following options: 
- Frogs
 - pigeons
 - gills
 - alveoli
 - fish
 - ferns
 - four limbs
 - water


 

    alveoli     