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A dam is a man-made structure built across a river. Most dams are built to control a river’s water flow, improve navigation and control flooding. However, some dams are built to produce hydro-electric power.
Hydro-electric power is produced as water passes through a dam, and into a river below. The more water that passes through a dam, the more energy is produced. Once a dam is built, a man-made lake is created behind the dam.
Electricity is produced by a kind of equipment called a turbine(涡轮机). Turbines contain metal coils(线圈)surrounded by magnets(磁铁). When the magnets move round rapidly over the metal coils, electricity is produced. Turbines are located inside dams. The falling water makes the magnets go around the coils.
Dams provide clean energy, but they can also harm the environment. Species that use rivers to reproduce are often hurt by dams. In the northwest of the US, the population of fishes has dropped from16million to2. 5million since hydro-electric plants were built on the Columbia River. Dams all over the world have hurt some species.
The highest dam in the United States is located near Oroville, California. The Oroville Dam towers230metres and is more than a mile wide. This dam was built in1968, 22years after the Hoover Dam. The Hoover Dam, on the Nevada-Arizona border controls the Colorado River.
It is221metres high and has2. 6million hectare-metres of water.
The highest dam in the world is the Nurek Dam on the Vakhsh River in Tajikistan, a country in central Asia. This dam is300metres tall. 【语篇概述】本文为说明文, 主要介绍大坝的定义、作用、利弊和水力发电的原理, 以及美国及世界上最高的大坝等知识。 Ⅰ. Skim the passage and find the main idea quickly.
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答案：The passage aims to introduce us a man-made structure—A dam and its advantages and disadvantages by giving examples. Ⅱ. Read the passage again and choose the best answers.
1. Which of the following is NOT the advantage of a dam according to the passage?
A. To control a river’s water flow.
B. To improve navigation and to control flooding. C. To reduce the population of some species of animals. D. To produce hydro-electric power.
【解析】选C。细节理解题。从第一段可知A、B、D三项正确, 从第四段的. . . but they can also harm the environment. Species that use rivers to reproduce are often hurt by dams. 可知造成某些动物物种数
量减少是大坝带来的负面作用, 故C项不对。 2. The third paragraph mainly tells us ________ . A. how hydro-electric power is produced B. what a turbine is and how it works C. how the magnets and the metal coils work D. how the falling water passes through a turbine
【解析】选A。主旨大意题。第三段主要告诉我们水电是怎样通过一些设备产生的, 故答案为A。
3. Which of the following can explain the underlined word “reproduce” in the fourth paragraph?
A. Produce young animals. B. Find foods. C. Live in.
D. Make a same photograph.
【解析】选A。词义猜测题。根据第四段In the northwest of the US, the population of fishes has dropped from16million to2. 5million since hydro-electric plants were built on the Columbia River. 可推知, 某些动物物种的数量在下降是因为建在河流上的大坝使得动物不能在水中繁殖, 故选A。