The Brainliest Answer!
They are called so because our lungs gets rid of harmful gases (carbon dioxide)and oxigenates our blood the plants also do the same thing.they take i  co2 and give o2 
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Plants give the purified air to the whole world same as how the lungs  supply purified and oxygenated air to the whole body.
 That is why plants called as earth lungs 
they also give the freshness and a new pure life to the earth.
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