Read the text and fill in the gaps with the correct form of verbs as per subject and context.

Air (a) (be) _____ an important element of the environment. But it is constantly being (b) _____ (pollute) in many ways. We make fire to cook food and (c) _____ (do) many works. This fire emits smoke which (d) _____ (contaminate) air. Vehicles pollute air to a great extent by (e) _____ (burn) fuel. Air pollution also (f) _____ (happen) when coal and oil (g) _____ (burn). Mills and factories can be (h) _____ (hold) responsible for this pollution. It is high time we (i) _____ (check) air pollution. Otherwise we (j) _____ (suffer) a lot.