Which description matches Earth's atmosphere given the composition 21% oxygen, 78% nitrogen, 1% other gases and water vapor?

• A. Ozone layer
• B. Lithosphere
• C. Earths atmosphere
• D. Hydrosphere

Answer: (C)

Atmosphere is the gaseous layer surrounding Earth, held in place by gravity. The air is mainly nitrogen (about 78%) and oxygen (about 21%), with small amounts of other gases and water vapor making up the rest. This matches the given composition, so it describes Earth’s atmosphere. The ozone layer is a part of the atmosphere that contains ozone and protects living things from UV radiation, but it isn’t the whole atmosphere. The lithosphere is the solid rocky crust, and the hydrosphere includes all Earth’s water. So the description corresponds to Earth's atmosphere.