B
Below is a question with multiple-choice answers. Choose the correct option based on your reasoning.

### Question:
Why are some rocks smoother and rounder than others?

### Choices:
A. They have the shapes of the minerals from which they are made.
B. They were smoothed by the action of wind and water.
C. They were squeezed up through the soil.
D. They hardened in rounded pockets of soil.

### Answer:
