0 a way of measuring the pollution (= dirty substances) in water, e.g. a river, by counting the organisms in it -- 生物指数
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。