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