In addition to the established energy sources such as gas, coal, oil and nuclear, there are a number of other sources that we ou

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问题     In addition to the established energy sources such as gas, coal, oil and nuclear, there are a number of other sources that we ought to consider. Two of these are hydroelectric and tidal power.
    These two sources are【C1】______in that they are both renewable.【C2】______, hydropower is more widely used than tidal. In【C3】______, a substantial amount of electricity is already produced in HEP (hydroelectric power)stations worldwide,【C4】______tidal stations are still in the very early【C5】______of development.
    As far as geographical【C6】______is concerned, HEP projects are to be found on lakes and rivers, while tidal【C7】______are constructed only at river mouths where tidal【C8】______is great. Unfortunately these are【C9】______in number. At present HEP stations are found mainly in Norway, Canada, Sweden and Brazil, whereas tidal plants are in【C10】______in France, Russia and China.
    As regards capital【C11】______, both require very high investment. On the other hand, generating【C12】______are quite low in both cases. In fact, a large scale HEP plant is capable of producing power more【C13】______than conventional sources, such as coal, oil and nuclear plants. Tidal power also compares【C14】______with nuclear and oil generated electricity,【C15】______the amount of money on production.【C16】______HEP stations, tidal constructions have a long life【C17】______. It is estimated that they can operate for over 100 years. With respect to【C18】______of supply, tidal stations【C19】______from HEP ones in that they often can only supply power【C20】______. HEP stations, however, provide a constant supply of electricity.
【C13】

选项 A、expensively
B、consistently
C、periodically
D、cheaply

答案D

解析 expensively昂贵地;consistently持续地;periodically周期性地;cheaply便宜地。该句及下一句说明这两种新能源电站与传统能源电站相比所具有的优越性,即耗费的成本低,故选D。
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