7. how much of earth's atmosphere is made up of nitrogen and oxygen?

The air is composed of several gases, the fundamental gases that form the atmosphere are Nitrogen by 78.084% and Oxygen by 20.946%. There are other gases present as Argon (0.934%) and Carbon Dioxide (0.033%). Other gases of interest present in the atmosphere are water vapor, ozone and different oxides.

C) to ensure each new cell has an exact copy of the dna

Gravitational force i believe