Oxygen is returned to the atmosphere mainly by

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

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

$(a)$ What do the blanks $1, 2, 3, 4$ and $5$ in the given cycle stand for?
$(b)$ Name two natural and one man-made process by which $CO_2$ returns to the atmosphere.
$(c)$ Carbon dioxide is necessary for plants but it is also a pollutant. Justify your answer.

Mention the three processes in which oxygen is used up from the atmosphere and the only process in which it is returned to the atmosphere.

Which step is not involved in the carbon cycle?