Pollution of rivers，lakes，and the oceans can come from many sources.Sometimes the source is （1）

easily

easily

（easy） located——it may be a pipe that discharges wastewater into a river，for example.Tighter regulations have almost got rid of this kind of pollution in some countries，（2）

while

while

it continues to be a problem in others.Discharges of harmful industrial waste into rivers can destroy life in water，and the release （3）

of

of

human waste into waterways can spread dangerous bacteria.
    In many cases， （4）

the

the

causes of water pollution are less specific.Wastewater from homes is subject to treatment before it reaches rivers，but water that runs off farmland，roads and gardens is not.It may contain a variety of pollutants，（5）

depending

depending

（depend） on its source.
    Water from farmland may contain fertilizers or pesticides.Fertilizers can cause uncontrolled growth of algae（海藻），（6）

which

which

use up the oxygen in the water，killing off fish and other organisms that require it.Pesticides may harm some forms of life in water directly.Other forms of pollution can result simply from
（7）

carelessness

carelessness

（careless） by people going about their everyday business.Chemicals—such as oil，cleaning products，and antifreeze-which have been improperly thrown away，or spilled and not cleaned up，can easily（8）

be washed

be washed

（wash） into rivers by rainwater runoff.
    Air pollution can also （9）

affect

affect

（effect）water quality.Sulfur dioxide and nitrogen oxides（氮氧化合物） react with rain-water，forming acids which can then accumulate in lakes，making the water acid.Some plants and animals in water may be badly affected because （10）

they

they

are very sensitive to changes in pH，or acidity-alkalinity.