You are presented with the quiz "how are caves formed?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "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". So, what's the best answer to the question?

Pick from:
1). less water.
2). eruptions.
3). Water expanding.
4). storing water.
5). volcanoes.
6). by humans.
7). chemical erosion.
8). hard outer covering.
Answer:
7).