You are presented with the question "Natural openings on the surface, leading to caves are formed by what?" and the following answer choices: 
- hydrogen and oxygen
 - aqueous solution
 - precipitation
 - calcite
 - erosion
 - flow of electrons
 - Calcium carbonate
 - Physical weathering

Now knowing that sinkholes are formed by precipitation and natural openings on the surface that lead to caves are called sinkholes, or swallow holes, choose the best answer.


 

   precipitation      