Answer the following question given this paragraph: 

It can be really exciting to explore a big underground cave like the one in this picture. Do you know how caves form? Believe it or not, water is the answer. Water slowly dissolves rocks, especially certain types of rocks such as limestone. How does something as mild and harmless as water cause hard rocks to dissolve? In this chapter, you’ll find out.


Q: Caves form over time when what causes rocks like limestone to dissolve?

 Choices:

- weather

- acid

- air

- water

A:
water
