You are presented with the question "what are formed by carbonic acid in groundwater seeping through rock and dissolving limestone?" and the following answer choices: 
- aqueous solution
 - barrel-shaped
 - Calcium carbonate
 - Greenhouse gasses
 - hidden caves
 - a solute
 - swamp vegetation
 - a hydraulic system

Now knowing that acavern is formed by carbonic acid in groundwater seeping through rock and dissolving limestone and caverns Secrets are hidden in caves, choose the best answer.


 

     hidden caves    