$(a)$ 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.

(N/A) Based on the carbon cycle diagram:
$1$ - Organic compounds (plants)
$2$ - Organic compounds (animals)
$3$ - Inorganic carbonates (shells)
$4$ - Carbonates in water
$5$ - Limestone
$(b)$ Two natural processes: Respiration and Decomposition. One man-made process: Combustion of fossil fuels.
$(c)$ $CO_2$ is necessary for plants for photosynthesis to produce food. However,it is a pollutant because:
$(i)$ It is a greenhouse gas that traps heat,leading to an increase in the average global temperature.
$(ii)$ Excessive $CO_2$ is the primary driver of global warming and climate change.