21. The carbon in coal, oil, and natural gas came from ________.

A. the combustion of fossil fuels
C. the remains of dead organisms
B. carbon-fixing bacteria in swamp soil D. carbon dioxide dissolved in ocean water

22. The water cycle is driven by ________.

A. Earth’s rotation
C. water
B. The sun
D. wind

23. Why are trees considered natural resources?
A. They provide valuable shade that no other resource can provide.
B. They are an important source of firewood for campfires.
C. Humans use them to produce paper, lumber, fuel, and other products.
D. They grow back after they're cut down.

24. Which of the following is the most sensible way to conserve natural resources?

A. Stop using electricity entirely.
B. Don't use any products made from trees.
C. Bike or walk to school instead of taking a gasoline powered car.
D. Never rides in a car.

25. How does recycling conserve natural resources?

A. It allows us to re-use products instead of manufacturing new ones.
B. It saves on the amount of fuel used by garbage trucks.
C. It prevents garbage from being burned.
D. It saves on the amount of plastic used to manufacture garbage bags.​