You are presented with the quiz "Which of the following added to groundwater creates caverns?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "acavern is formed by carbonic acid in groundwater seeping through rock and dissolving limestone" and "any time water and carbon dioxide mix, carbonic acid is the result". 

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


 

       carbon dioxide    