State whether the following statement is True or False:
Nitrogen gas makes up $78 \%$ of our atmosphere and nitrogen is also a part of many molecules essential to life like proteins,nucleic acids ($DNA$ and $RNA$) and some vitamins.

State whether the following statement is True or False:
Nitrogen is also found in other biologically important compounds such as alkaloids and urea.

What is nitrogen fixation? Why do plants need to fix nitrogen?

What are the three ways by which $CO_{2}$ is returned back into the atmosphere?

$(a)$ Study the given figure of the nitrogen cycle and mention what $A, B, C, D,$ and $E$ represent.
$(b)$ What will happen if step $A$ does not occur?
$(c)$ Write the role of nitrogen-fixing bacteria in the biosphere.
$(d)$ Name two biologically important compounds that contain both $O_2$ and $N_2$.

Oxygen is returned to the atmosphere mainly by