完型：At the same time,a steel pipe(钢管) is p______ down to .A.put B.pulled C.pushed

完型：At the same time,a steel pipe(钢管) is p______ down to .A.put B.pulled C.pushed
At the same time,a steel pipe(钢管) is p______ down to stop the sides from falling in and to keep out water.
附全文：Nowadays more and more people care about the exploitation(开采) of the natural resources,such as oil.Though there are many kinds of resources,with the fast increase of population and appearance of many more factories,the more we use,the fewer there will be left for the future.Oil is getting less and less year after year.It's said that oil can last for only 50 years.Natural gas perhaps will last about 38 years.
Oil can be used in many ways and it's difficult to imagine what the modern world will be like without oil.But oil isn't easy to find and get out of the earth.Men must study the rocks.When they think the rocks in a certain place may have oil,a metal tower is built.A machine in the tower cuts a hole down into the ground.At the same time,a steel pipe(钢管) is p_______down to stop the sides from falling in and to keep out water.At last if the men are right,usually the oil rushes up the sides with great force(力量) by pressure(压力)of the gas in the top of the rock,and it rushes high into the air.If oil catches a light,there will be a terrible fire.So a kind of cover is fixed on the top of the pipe,and the oil can run out through taps.(活塞)
If we make a well near the middle of the oil field,we can also get gas.Such gas is sent through pipes to towns far away and used in houses and factories like coal gas.
Today oil is under pressure as never before.The price of oil keeps rising all the time.In many countries,the govenments encourage people to save oil as much as possible.