You are presented with the quiz "A breathing mask can prevent what kind of particle from entering the lung?" 

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 "unlike larger particles, the smaller particles are able to enter the respiratory system". 

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


 

        small   