You are presented with the question "What forms caverns by seeping through rock and dissolving limestone?" and the following answer choices: 
- pure oxygen
 - Something with a head, thorax, and abdomen
 - basic building blocks of life
 - carbon dioxide in groundwater
 - magma in groundwater
 - oxygen in groundwater
 - At the peak of a mountain
 - underground systems

Now knowing 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, choose the best answer.


 

    carbon dioxide in groundwater     