Traditional sources of energy such as oil are becoming more expensive and difficult to findthese days. At the same time, modern society's dependence on energy is increasing significantlyeach year. Alternative sources such as solar, wind and nuclear power are being utilized to fill someof those needs. Recently, however, scientists and businesspeople have begun to experiment with yetanother possible source of energy—tidal power. Tides, the rising and falling of the oceans' waterlevels, are caused by the pull of the sun and the moon on the water. Tidal power makes use of thisnatural movement to facilitate the production of electricity.
            There are two basic ways of collecting energy from the movements of the tides. The first usesenergy from the actual movement of the water as levels rise and fall. Machines called turbines workalmost like the wind towers used to generate electricity. Each turbine has blades on the outside thatare driven round by the moving water. However, they are different from wind-powered devices inthat they operate half of the time in one direction (as the tide comes in) and the other half of thetime in the opposite direction (as the tide goes out). The second way of getting power from tidalmovements involves making use of the fluctuation in water level between high tide and low tide. Asthe tide rises, water fills up a special catchment area called a reservoir near the shore. As the tidebegins to reverse direction, the doors of the reservoir close, capturing the water there. The water islater released through a passage with a turbine in it which converts the water power into electricity.
             Advocates for tidal power point out that once a station has been built, the power it generatesis basically free. No additional fuel sources are required to keep it operating. Furthermore, tidalpower doesn't produce any dangerous gases or other waste and the equipment is very inexpensive tomaintain. A final plus is that, unlike solar power and wind power, tidal power is totally reliable. Theweather can be temporarily cloudy and sometimes the wind doesn't blow. However, the tides areguaranteed on to keep up their powerful, steady movements 365 days a year.
           A major disadvantage of tidal power stations is that they are very expensive to build. Asresearchers gain experience and technological advances are discovered, this will undoubtedlychange. A second problem is that this process works best in places where the difference betweenhigh tide and low tide is very large. There are a finite number of such places in the world whichmeans there is a limit to how many stations can be built. A third concern involves environmentalimpact. Early experiments have shown that the machinery used can sometimes harm local plant andanimal life. Researchers are currently working on ways of making the stations moreenvironmentally friendly.
【題組】48. This reading is mainly about ______.
(A) the future of tidal power
(B) how tidal power compared with other types of power sources
(C) the current situation regarding tidal power
(D) the energy demands of modern society