You are presented with the quiz "The emission of what causes acid rain?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "the emission of sulfur dioxide causes acid rain" and "sulfur Dioxide and Asthmatics A highly reactive colorless gas smelling like rotten eggs, sulfur dioxide derives primarily from fossil fuel combustion". 

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


 

     Something that smells like rotten eggs      