You are presented with the question "What can be formed by carbonic acid in groundwater seeping through rock and dissolving limestone?" and the following answer choices: 
- water
 - swamp vegetation
 - Calcium carbonate
 - sunlight
 - contamination
 - loose soil
 - underground caves
 - arsenic

Now knowing that acavern is formed by carbonic acid in groundwater seeping through rock and dissolving limestone and underground caves and caverns pose an exciting adventure to many, choose the best answer.


 

       underground caves  