You are presented with the quiz "what are formed by carbonic acid in groundwater 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 "caverns Secrets are hidden in caves". 

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


 

     hidden caves      