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Answer Key
REINFORCING KEY CONCEPTS
1. a. nitrogen; b. oxygen; c. argon, carbon dioxide, and other
gases. Why we need it: protects Earth from radiation, keeps
Earth warm, transports energy.
2. Carbon cycle: Animals inhale air and exhale carbon dioxide.
Plants take in carbon dioxide and release oxygen into the air.
Nitrogen cycle: Tiny organisms remove nitrogen gas from the
air and change it into solid chemicals that enter the soil. Plants
and animals use nitrogen, which is returned to the soil when
they die and decay. Nitrogen is returned to the air from the soil.
Water cycle: Water in oceans and lakes changes into gas and
enters the atmosphere. Plants release water vapor from their
leaves. Water falls from the atmosphere as precipitation.