听力题 Today I would like to continue our discussion of energy sources. Scientists believe a great energy source of the future can be found on the world''s oceans. There are several systems being studied which use power from the waves, tides and other sources. Since most of these projects are still in the experimental stage, they are not cheap, but the cost is expected to go down in the future. These systems seem to cause little or no negative impact on the environment, unlike conventional sources such as oil and coal, and there is no lack of seawater in the world. One example of a new energy source using the world''s oceans is tidal power, the rhythmic rise and fall of ocean water passing through the turbines of a dam built along the coast can be used to produce electricity. However, the range of tides necessary for an economically worthwhile system is found in only a few coastal areas of the world, such as in France and in the United States. Experts believe that tidal energy causes little damage, but the long range effect may be to slow down the rotation of the earth very slightly.