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  3. In modern industry, water fulfills several essential functions. It is used as a raw material in many processes; it is essential

In modern industry, water fulfills several essential functions. It is used as a raw material in many processes; it is essential

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问题     In modern industry, water fulfills several essential functions. It is used as a raw material in many processes; it is essential for cooling machinery; it provides steam for engines which consume coal and it is one of the most important sources of power.
    As a source of power, water is virtually inexhaustible. Its power is ultimately derived from the force of gravity and from the sun. It is the sun’s energy that warms the sea and the land and causes water to evaporate into the air; later the moisture in the air condenses and falls in the form of rain or snow. Much of the water that falls on the land flows back to the sea. The downward flow of the water exerts considerable pressure, and it is at this point in the water cycle that we can capture some of its power.
    The earliest method of exploiting this power was to make water turn a wheel. This system in which the force of water was used directly, had one great disadvantage. Factories which used it had to be built on the banks of fast flowing streams, but these were often located in inaccessible, thinly populated areas, which made transportation of goods difficult.
    When the steam engine was invented, it soon replaced flowing water as a source of power, new factories sprang up in the coal fields. Coal and oil—reserve of power also ultimately derived from the sun—are not always found in accessible places, but they have the advantage of being portable.
    In the 20th century the situation changed and water once more became a vital source of power. Methods were devised for producing electricity from the energy of rivers. Once the initial capital has been recovered, electricity from this source is usually cheaper than oil and coal, which are expensive to extract and transport. The great advantage of hydroelectric power is that nature constantly renews the water that provides the power. Coal and oil are fuels that can only be used once.
    Not all rivers are ideal for generating electricity. A suitable river must have an adequate flow of water, and ideally there should be a steep fall. A steep waterfall is ideal, because it concentrates the energy of gravity into a short distance. The amount of water in the river, moreover, should not vary too much from season to season. This means that there must be adequate rainfall throughout the year. The river must be fairly near the industrial centers which will utilize its power, for the greater the distance the electricity has to travel, the more power is wasted.
    Waterfalls can be constructed or enlarged by engineers. This is done by altering the slope or level of rivers at certain places, in order to concentrate a heavy fall of water at a chosen location. An artificial waterfall thus obtained is usually regulated by a dam. Projects of this kind, harnessing the flow of rivers are in process of construction in Egypt, Australia, and many other countries.
    Japan is a country that relies on hydroelectric power. Her available coal resources proved inadequate for her industrial development and she was obliged to import fuel to drive machinery. But Japan is a mountainous country with abundant rainfall and a dense concentration of population, she has developed her resources of hydroelectricity to a greater extent than other Asian countries.
选项 A、when the steam engine was invented, it soon replaced water as a source of power, so water was useless for a certain period
B、Factories could be built far from coal fields, because coal is portable
C、It was convenient to build a factory in the coal fields
D、Japan is a country that relies on hydroelectric power
答案A
解析 辨析题。全文都在讲述水是一种重要的动力源，因此A与全文思想相反，其它三项均在第四段中提到。
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