题目内容：

阅读理解。

A dam is like a wall that is built across a waterway to control the flow of the water.For many years,dams were built only of earth and rocks.Today,many dams are built of concrete.

When a dam is built,water is trapped and held behind the dam.This water forms a lake.The lakes can be used for recreation or for irrigating,or watering crops.The lakes also provide potable water,water that is safe for drinking.This potable water is piped from the lake into houses and other buildings.The lake water further serves as a home,or habitat,for fish and waterfowl.

Dams are often built to provide flood control.Just before the rainy season,the water in the dam is allowed to flow out very slowly.This lowers the water level in the lake so that snowmelt and rainfall can be stored there in every spring and used as needed during the rest of the year.

Some dams are built to use the power of falling water to make electricity.Such dams have powerhouses that contain machines;turbines and generators.Water falls from the lake to the river below the dam.As the water falls,it is sent through the powerhouse.There,the force of the water turns the turbines and generators and produces electrical power needed to light and heat buildings.

1.The word used to describe water that is safe for drinking is    .

A.portable water           B.dirty water

C.orange juice            D.boiled water

2.According to the story,dams are built for different reasons.But all dams    .

A.control the flow of water

B.are built of earth and rocks

C.provide electrical power

D.provide potable water

3.If the water level in the lake behind the dam is not lowered before the rainy season,the    .

A.water in the dam will flow out too slowly

B.snowmelt and rainfall might cause a flood

C.lake will probably dry up

D.water can irrigate

4.When dams are built to provide electricity,which of the following happens first?Water    .

A.is sent through the powerhouse

B.pressure turns the turbines and generators

C.falls from the lake to the river below the dam

D.runs fast