In addition to the established energy sources such as gas, coal, oil and nuclear, there ar

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.

【21】

A．compatible

B．parallel

C．similar

D．identical