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. Given these facts, how are caves formed among the following options:
- less water
 - eruptions
 - Water expanding
 - storing water
 - volcanoes
 - by humans
 - chemical erosion
 - hard outer covering


 

       chemical erosion  