1. When is wind an agent of erosion?
a. when it breaks rocks into pieces
b. when it carries rock pieces to other places
c. when it stops picking up rocks
d. when it presses down on rocks

2. How can trees help prevent erosion?
a. the roots hold the soil firmly together
b. the trees block the way of running water
c. the roots absorb the water at once
d. the roots secrete acid, attacking the rocks

3. What makes the water muddy after a heavy rain?
a. the pebbles picked up by water
b. the rainwater turned brown in color
c. the gravel and sand carried by the water
d. the fine dust particles dissolved in water