单项选择题

Interest is steadily spreading from a minority of enthusiasts in developing renewable sources of energy—wind, wave and solar power, tidal and geothermal energy. Additional support for them has come with a proposal to explore the untapped sources of hydro-electric power in Scotland.
The details are presented by Mr. William Manser in a study called "The Case for an Inquiry into Hydro-Electric Generation in the North of Scotland" He calls for an expert committee to look at the developments possible for hydro-electric sites and, more important, for means of financing them.
There is a clear industrial connection in Mr. Manser’s study because it was done for the Federation of Civil Engineering Contractors; hydro-electric schemes, by definition, have a large civil engineering component in them. Mr. Manser estimates that wind power could theoretically provide more than 7 per cent of electricity supply in the United Kingdom, provided suitable sites for generators could be found. However, the practical viability of wind power generation is not likely to be understood until 1990.
Other developments using renewable energy sources are also at an early stage as far as their commercial possibilities are concerned, he believes.
The best developed and most suitable form of renewable energy is, in his view, hydro-power. The technology has been developed over centuries and is still progressing. At present it is the cheapest form of electricity generation.
Mr. Manser examined past surveys of the north of Scotland and identified several as suitable for hydro-electric generation. Those are in the remote areas, usually of great natural beauty.
But Mr. Manser says a well-designed dam can be impressive in itself. It is also possible to make installations as unobtrusive as possible, to the point of burying parts of them. Hydro-generation involves no water pollution, smoke creation or unsightly stocking-out yards.
The main trouble, appearing from his report, is financing an undertaking which has a heavy initial capital cost, and very low running costs.
However, Mr. Manser does not see that as an unfamiliar position for the electricity industry. He cites the proposed construction of the new nuclear power station at Sizewell in Suffolk, which will have a high initial capital cost.
The argument as Sizewell that the reason for the expenditure is that the capital will provide a benefit in lower costs and higher returns in the long-term, applies equally to hydro-electric generation.
The advantage of hydro-electric power is that it ______.