Which statement describes a simple model of the Earth's atmosphere?
It covers the Earth to a height of about 500 km500\text{ km}500 km and is of uniform density.
It covers the Earth to a height of about 500 km500\text{ km}500 km and its density increases as the distance from the Earth increases.
It covers the Earth to a height of about 500 m500\text{ m}500 m and is of uniform density.
It covers the Earth to a height of about 500 m500\text{ m}500 m and its density increases as the distance from the Earth increases.