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...resource. If polluting discharges to a river are intermittent, the river is often able to return to a clean and unpolluted condition as the pollutants are flushed out and carried down to the sea. In addition, because of the organisms present (e.g. bacteria capable of breaking down organic matter), river water has some capacity for self-purification – unless too many of...

...human eye, as in previous units in this series. Flow charts of the type you drew for yourself earlier express an idea, in the form of a relationship between different components. They encapsulate the key information that might otherwise take many hundreds of words to describe - why not attempt to use one to summarise an idea or a section of text at this point? Other...