Question

explain why and how carbon dioxide is necessary for plants. why do we also consider it a pollutant

Answer 1

CO2 is necessary for plants as it is required during the process of photosynthesis as plants intake CO2 for making its food and release oxygen which is required by human.
it is pollutant as co 2 us released from burning of fossil fuels.

Answer 2

it is it is true that carbon dioxide is necessary for plants as they need it to synthesise carbohydrates during photosynthesis but excessive amount of Co2 in atmosphere becomes harmful for living organism and hence it is considered as pollutant.

Excess of excess of Co2 in atmosphere is causing global warming by forming an annual up around the earth which prevent heat from the Earth surface to escape into the atmosphere.