You are presented with the quiz "What forms caverns by seeping through rock and dissolving limestone?" 

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 "when carbon dioxide is in water, it creates carbonic acid". 

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


 

    carbon dioxide in groundwater       