Fact 1: Acavern is formed by carbonic acid in groundwater seeping through rock and dissolving limestone.

Fact 2: Erosion by acid solutions is only one way that caves form.

Given the two facts above, answer the question "how are caves formed?" with the following options: 
- less water
 - eruptions
 - Water expanding
 - storing water
 - volcanoes
 - by humans
 - chemical erosion
 - hard outer covering


 

       chemical erosion  