But this challenge has not prevented scientists from trying.Within the last few decades，several technologies that can transform the ocean’s huge forces into usable electricity have been invented and introduced.Some focus on capturing the power of the changing tides，while others rely on hot energy created by oceans in certain tropical region.However，the most common and easiest technology to develop is that designed to use the power in the ocean’s waves.

　　The potential benefits of wave－based energy are hard to ignore.Once the proper machinery is produced and fixed，the energy is free.Maintenance costs are small and the equipment does not pose any threats of environmental pollution.And best of all the amounts of energy produced are enormous.

　　However，these theoretical advantages have yet to be fully realized.In many cases，a lack of government funding has prevented the technologies from advancing.Protecting the equipment from the sea’s destructive forces，as well as the fundamental task of determining appropriate locations for collecting energy，also presents difficult challenges.All in all，while ocean power offers some attractive possibilities，the difficulties involved in using this energy source are substantial and will require more time to overcome.

　　1.____________

　　of

　　sustainable energy The power of the sun, the wind,2.____________

　　and the oceans

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