You are presented with the question "What does a gas mask keep out?" and the following answer choices: 
- disease
 - The atmosphere
 - heat energy
 - air pollution
 - energy barrier
 - Bodily water
 - Dehydration
 - bacteria

Now knowing that abreathing mask is used for keeping harmful particles from entering the respiratory system and air pollution is caused by the build-up of harmful gases and particles in the air, choose the best answer.


 

    air pollution     