Which of the following is not a greenhouse gas?

Name two chemicals present in living organisms that have carbon,hydrogen,and oxygen as their main constituents. State their main function.

State whether the following statement is true or false:
Oxygen is returned to the atmosphere in only one major process,that is,respiration.

Explain the formation of soil. What is top soil? Why is it important?

When we breathe in air,nitrogen also goes inside along with oxygen. What is the fate of this nitrogen?

$(a)$ 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 $N_2$.