If hidden caves are formed by carbonic acid in groundwater seeping through rock and dissolving limestone, then what are formed by carbonic acid in groundwater seeping through rock and dissolving limestone?

Answer choices:
- aqueous solution
 - barrel-shaped
 - Calcium carbonate
 - Greenhouse gasses
 - hidden caves
 - a solute
 - swamp vegetation
 - a hydraulic system

 

     hidden caves     