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  3. When it rains or snows, some of the water is reserved by the soil, some is absorbed by vegetation, some evaporates, and the rema

When it rains or snows, some of the water is reserved by the soil, some is absorbed by vegetation, some evaporates, and the rema

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问题    When it rains or snows, some of the water is reserved by the soil, some is absorbed by vegetation, some evaporates, and the remainder, which reaches stream channels, is called runoff(径流). Floods occur when soil and vegetation cannot absorb all the water; water then runs off the land in quantities that cannot be carried in stream channels or retained in natural ponds and constructed reservoirs. About 32 percent of all precipitation (雨雪的降落) is runoff, and this amount may be increased by melting snow masses. Periodic floods occur naturally on many rivers, forming an area known as the floodplain (涝原). These fiver floods often result from heavy rain, sometimes combined with melting snow, which causes the rivers to overflow their banks; a flood that rises and falls rapidly with little or no advance warning is called a flash flood (暴洪). Flash floods usually result from intense rainfall over a relatively small area. Coastal areas are occasionally flooded by unusually high tides induced by severe winds over ocean surfaces, or by tidal waves (海啸) caused by undersea earthquakes.
   Floods not only damage property and endanger the lives of humans and animals, but exert other effects as well. Rapid runoff causes soil erosion as well as sediment (沉淀物) deposition problems downstream(下游). Spawning(产卵) grounds for fish and other wildlife habitat are often destroyed. High-velocity currents increase flood damage; prolonged high floods delay traffic and interfere with drainage and economic use of lands. Bridge abutments (桥台) , bank lines, sewer outfalls, and other structures within floodways are damaged, and navigation and hydroelectric power are often impaired(侵害). Financial losses due to floods all over the world are commonly billions of dollars each year. The basic methods of flood control have been practiced since ancient times. These methods include reforestation (再造林) and the construction of levees(防洪堤), dams, reservoirs, and floodways.  
To control floods, people usually ______ .
选项 A、plant more trees and construct levees and reservoirs
B、fill the ponds with rocks and stones
C、damage the floodways
D、get warnings in advance
答案A
解析 文中最后一句话有提及。C项意为“(人们通常)破坏导洪渠”。B、C、D三项均为干扰项。
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