17. ________________ could increase the amount of oxygen in the air.

A. Building more houses
C. Burning more oil
B. Paving more streets
D. Planting more trees

18. Plants use _____________ to make their own food during photosynthesis.

A. CO2
C. water
B. energy from the Sun
D. water, energy from Sun, CO219.

What is the largest reservoir for water?

A. glaciers
C. the ocean
B. the atmosphere
D. rocks and minerals

20. In an ecosystem, what happens to the atoms of certain chemical elements, such as carbon, oxygen, and nitrogen?

A. They move into and out of living systems.
B. They move out of living systems and do not return.
C. They are only found in abiotic factors.
D. They move into living systems and remain there.​