You are presented with the quiz "Breathing masks are used to keep harmful particles from entering the mouth and the what?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "abreathing mask is used for keeping harmful particles from entering the respiratory system" and "oxygen enters the respiratory system through the mouth and the nose". 

So, what's the best answer to the question?


 

      nose     