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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(开采) 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.
26. If you want to make water turn a wheel, you should keep in mind the difficulty to
A. use it directly B. carry goods out
C. cross the fast flowing streams D. find enough workers
27. The steam engine soon replaced flowing water because_______.
A. coal and oil are derived from the sun
B. coal and oil are not always accessible
C. coal and oil are portable
D. coal and oil are found in thinly-populated areas
28. After using coal and oil for a time, people find the advantages of water over coal and oil as_______.
A. it is easy to recover the initial capital
B. it is as cheap to use water as to use coal and oil
C. it is nature that renews the water without any cost
D. it is difficult to extract and transport coal and oil
29. All of the following are required for an ideal river EXCEPT that_______.
A. there should be energy of gravity
B. there should be a steep fall
C. the river should be near industrial centers
D. there should be an adequate flow of water
30. It can be inferred from the passage that if people want to establish a water power station, they will most probably go to_______.
A. the mountainous areas B. the plains
C. the lake areas D. the river mouths 答案：26. B 27. C 28. C 29. A 30. A
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