Fact 1: Sinkholes are formed by precipitation.

Fact 2: Natural openings on the surface that lead to caves are called sinkholes, or swallow holes.

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


 

   precipitation      