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

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【21】______ in that they are both renewable. 【22】______ , hydropower is more widely used than tidal. In【23】______ , a substantial amount of electricity is already produced in HEP (hydroelectric power)stations worldwide, 【24】______ tidal stations are still in the very early【25】______ of development.
    As far as geographical【26】______ is concerned, HEP projects are to be found on lakes and rivers, while tidal【27】______ are constructed only at river mouths where tidal【28】______ is great. Unfortunately these are【29】______ in number. At present HEP stations are found mainly in Norway, Canada, Sweden and Brazil, whereas tidal plants are in【30】______ in France, Russia and China.
    As regards capital【31】______ , both require very high investment. On the other hand, generating【32】______ are quite low in both cases. In fact, a large scale HEP plant is capable of producing power more【33】______ than conventional sources, such as coal, oil and nuclear plants. Tidal power also compares【34】______ with nuclear and oil generated electricity, 【35】______ the amount of money on production. 【36】______ HEP stations, tidal constructions have a long life【37】______ . It is estimated that they can operate for over 100 years. With respect to【38】______ of supply, tidal stations【39】______ from HEP ones in that they often can only supply power【40】______. HEP stations, however, pro vide a constant supply of electricity.  
【24】
选项 A、provided
B、since
C、whereas
D、though
答案C
解析 provided可作连词表示假如；since既然；whereas(用以比较或对比两个事实)然而，但是，尽管；though虽然。这里对水力发电站和潮汐电站的发展进行了对比，故选C。
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