I would say Carbon, Hydrogen and Oxygen.
9000/10=900 people per square mile
Surprisingly, there are 196 countries today! hope this helped
Earth’s atmosphere is mainly composed of nitrogen (N) and oxygen (O).
This is further subdivided into four layers according to their temperatures.
Troposphere the layer closes to the Earth's surface with 10-15 km above its
surface. Stratosphere is 15 to 50 km above the surface of the Earth and its top
is warmer than its bottom. Mesosphere extends 50
to 80 km and becomes colder with increasing altitude. <span>And thermosphere is
80 km above the surface of the Earth to outer space and is very hot.</span>
