Dams are classified by the type of material used in their construction and by their shape．Dams call be constructed from concrete，stone，loose rock，earth，wood，metal，or a combination of these materials．Engineers build dams of different types，depending on the conditions of the riverbed and the surrounding landshape，and the construction materials．

    Gravity dams use only the force of gravity to resist water pressure — that is，they hold back the water by the force of their weight pushing downward．They are much thicker at the base than the top —a shape that reflects the distribution(分布)of the forces of the water against the dam．As water becomes deeper，it puts more pressure on the dam．Gravity dams are relatively thin near the surface of the reservoir，where the water pressure is light．

    An embankment dam is a gravity dam formed out of loose rock，earth，or a combination of these materials．

    Arch dams are concrete or stone structures that curve upstream into a reservoir，requiting a relatively narrow fiver canyon(峽谷)with solid rock walls able to support a large amount of force．These dams do not need to be as massive as gravity dams because the canyon walls carry part of the pressure from the reservoir．Because．they require less material than gravity dams，arch dams can be less expensive to build．

    A buttress dam consists of a wall，or face，supported by several buttresses every 6 to 30 meters on the downstream side．Buttress dams are sometimes called hollow dams because the buttresses do not form a solid wall stretching across a fiver valley．It falls into two basic kinds：flat slab and multiple arches．Flat slab buttress dams have a flat upstream face．Multiple arch buttress dams have an upstream face formed by a series of arches．

    Like arch dams，buttress dams require less concrete than gravity dams，but they are not necessarily less expensive to build than gravity dams．Costs with the work of forming the buttresses or multiple arches may offset(抵消)the savings in construction materials．Buttress dams may be desirable，however，in locations with foundations that would not easily support the massive size and weight of gravity dams．

1．Which of the following pictures best stands for gravity dams?

2．According to the passage，it generally costs less to build _______．

      A．a(chǎn)rch dams than gravity dams             B．buttress dams than gravity dams

      C．gravity dams than arch dams             D．gravity dams than buttress dams

3．Which of the following would be preferred to build in places with foundations not strong enough?

      A．Gravity dams．                              B．Arch dams．

      C．Buttress dams．                              D．Embankment dams．

4．The best title for this passage would be _________．

A．Functions of Dams                           B．Constructions of Dams

C．Costs of Dams                              D．Types of Dams

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A．Australians are a suburban people

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Its big cities lie on the southeast coast, this is where most Australians live. Australians prefer to own their own houses, though some live in apartments. Australians are a suburban（郊區(qū)的） people. The suburbs surround the cities for many miles, and so efficient transport is of great importance. As the economy grows, so do its industries- a higher level of production, a wider range of products.

   The Australian works hard, but he likes his leisure. The climate makes outdoor activities the most popular.

   Canberra, the capital of Australia, is a planned modern city located inland. Australia is governed by a parliamentary democracy(議會(huì)民主). The representatives of other countries have their embassies here. Australia wants to strengthen relations with her neighbors.

Australia is a strange land, a land of vast expanses- fertile valleys, snow fields and deserts- also a land with unique animal, many that can not be found on any other continent in the world today.

Much of the continent is dry, but man has utilized the land, made it productive, with its tools, with its technology. This is the driest continent of all, and water is a precious possession, more precious than all other natural resources. Large dams are built to collect the water, there to irrigate the fields of pastures（牧場(chǎng)） and crops.

But Australia is changing. The land of wool and wheat is now a land of large-scale industry and mining. The costs of developing the new mineral discoveries are enormous, but the rewards are great too.

Australia — a young and developing nation. Australia — a nation that wants to communicate with its neighbors.

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A. to the south; on the southeast             B. to the north; on the southwest

C. to the east; on the northeast          D. to the east; on the southwest

2.We can infer from the passage that the Australian likes outdoor activities for the _____ climate.

A. dull and wet    B. fine and shiny  C. gloomy and rainy                 D. wet and cold

3.____ is the most precious source in Australia.

A. Mineral resource                       B. Animal         C. Desert                  D. Water

4.Which of the following statements about Australia is wrong?

A. Australians are a suburban people

B. Australia is governed by a parliamentary democracy.

C. Australia prefers to live in the downtown of big cities.

D. Wool and wheat used to be the main products of Australia.

 

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