The prairie grass ecosystem once had a deep layer of topsoil which was protected by the grasses that covered it. Removal of these grasses for farmland is causing the soil to be eroded mainly by OPTIONS:
- wind and rain.
- animal movement.
- crops grown in the soil.
- increased temperatures.
A: wind and rain.

Infections may be caused by OPTIONS:
- mutations
- microorganisms
- toxic substances
- climate changes
A: microorganisms

Fossil fuels are considered a nonrenewable resource because they OPTIONS:
- have turned into solid rock.
- take a long period of time to form.
- are expensive to remove from rock.
- are located deep within the Earth's crust.
A: take a long period of time to form.

Nuclear fusion is the source of energy for stars. Besides hydrogen, which other element is most likely also common in stars? OPTIONS:
- helium
- nitrogen
- oxygen
- uranium
A:
helium