Q: You are presented with the quiz "Natural openings on the surface, leading to caves are formed by what?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "sinkholes are formed by precipitation" and "natural openings on the surface that lead to caves are called sinkholes, or swallow holes". So, what's the best answer to the question?

Available choices:
1). hydrogen and oxygen
2). aqueous solution
3). precipitation
4). calcite
5). erosion
6). flow of electrons
7). Calcium carbonate
8). Physical weathering
A:
3).