题目内容：

Each year, road accidents kill a million people and injure millions more . The economic costs are greatest for developing countries . Earlier this year , the United Nations called for a campaign to improve road safety .

One way to avoid accidents is better driving . Another is better roads and bridges . Engineers in the Unites States have designed ten new concrete mixtures that they think could make bridges last longer .

Professor Paul Tikalsky leads the experiments by a team at Pennsylvania State University . He says bridges made of concrete now last about twenty-five to thirty-five years . But he says the new mixtures might extend that to seventy –five or even one-hundred years .

Concrete is made of stone , sand , water and cement(水泥). The materials in the cement hold the concrete together . Ancient Romans built with concrete . Yet strengthened concrete bridges did not appear until the late 1800s . People keep looking for new ways to improve concrete . Professor Tikalsky says it is one of the most complex of all chemical systems .

The new mixtures designed by his team contain industrial waste products . He says these make the concrete better able to resist damage from water and salt over time . One of the products is fly ash . This is released into the air as pollution when coal is burned .

Professor Tikalsky says particles (颗粒)of fly ash are almost exactly the same size and chemical structure as Portland cement . This is the most costly material in concrete . So using fly ash to replace some of it would save money .

The federal government is paying for part of the research . Engineers anywhere can use the technology . Professor Tikalsky says some of the ideas have already been put to use in China , the Philippines and other countries .

1.Why did the United Nations launch the campaign ?

A．Because the United Nations wanted to reduce road accidents and economic costs

B．Because two million people were killed in the accidents .

C．Because engineers wanted to design ten new concrete mixtures .

D．Because the United Nations made bridges of the new mixtures which could last for about 20 to 35

years .

2.What does the underlined word “ this ” in the sixth paragraph refer to ?

A．Fly ash

B．Portland cement

C．Sand

D．Chemical

3.Which of the following statements is TRUE ?

A．Ten new concrete bridges have already been built in the United States .

B．A new concrete bridge could last 50 more years than an ordinary concrete one .

C．People didn’t know how to build with cement until the late 1800s .

D．Water and salt won’t do any damage to bridges over time .

4.What does the passage mainly tell us ?

A．The causes of road accidents .	B．The advantages of fly ash
C．The measures of avoiding road accidents	D．Latest information about long-life concrete bridges