0 relating to rules that limit what vehicles can be used, what fuels can be burned, etc., in order to reduce the amount of pollution in the air -- (燃料等)清洁的,不污染空气的
We can complete it step by step however long the road is and it can't be completed however short the road is if you don't even mark your footprint.
再长的路,一步步也能走完,再短的路,不迈开双脚也无法到达。