You are presented with the question "What causes acid rain?" and the following answer choices: 
- Pesticides
 - fossil fuels
 - remains of prehistoric life
 - chlorofluorocarbons
 - energy of moving objects
 - an erupting volcano
 - harmful substances
 - forest is destroyed

Now knowing that the emission of sulfur dioxide causes acid rain and an erupting volcano emits sulfur dioxide, choose the best answer.


 

      an erupting volcano   