You are presented with the question "how are caves formed?" and the following answer choices: 
- less water
 - eruptions
 - Water expanding
 - storing water
 - volcanoes
 - by humans
 - chemical erosion
 - hard outer covering

Now knowing that acavern is formed by carbonic acid in groundwater seeping through rock and dissolving limestone and erosion by acid solutions is only one way that caves form, choose the best answer.


 

       chemical erosion  