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What are the best energy sources? "Best" depends on many factors - how the energy is being used, where it is being used, what energy sources are available, which sources are most convenient and reliable, which5 are easiest to use, what each costs, and the effects on public safety, health, and the environment. Making smart energy choices means understanding resources and their relative costs and benefits. Some energy sources have advantages for specific10 uses or locations. For example, fuels from petroleum are well suited for transportation because they pack a lot of energy in a small space and are easily transported and stored. Small hydroelectric installations are a good solution for supplying power or mechanical energy close15 to where it is used. Coal is widely used for power generation in many fast-developing countries - including China, India, and many others - because domestic supplies are readily available. Efficiency is an important factor in energy costs.20 How efficiently can the energy be produced, delivered, and used? How much energy value is lost in that process, and how much ends up being transformed into useful work? Industries that produce or use energy continually look for ways to improve efficiency, since this is a key to25 making their products morecompetitive. The ideal energy source - cheap, plentiful, and pollution-free - may prove unattainable in our lifetime, but that is the ultimate goal. The energy industry is continuing to improve its technologies and practices, to30 produce and use energy more efficiently and cleanly. Energy resources are often categorized as renewable or nonrenewable. Renewable energy resources are those that can be replenished quickly - examples are solar power,35 biomass, geothermal, hydroelectric, wind power, and fast-reaction nuclear power. They supply about seven percent of energy needs in the United States; theother 93 percent comes from nonrenewables. The two largest categories of renewable energy now in use in the U.S.40 are biomass - primarily wood wastes that are used by the forest products industry to generate electricity and heat - and hydroelectricity. Nonrenewable energy resources include coal, oil, natural gas, and uranium-235, which is used to fuel45 slow-reaction nuclear power. Projections of how long a nonrenewable energy resource will last depend on many changeable factors. These include the growth rate of consumption, and estimates of how much of the remaining resources can be economically recovered. New exploration50 and production technologies often increase theability of producers to locate and recover resources. World reserves of fossil energy are projected to last for many more decades - and, in the case of coal, for centuries.In: http://www.classroom-energy.org/teachers/energy_tour/pg5.html
A "pollution-free" energy source (line 27) means an energy source that is free of pollution. Check the option in which the phrase is also correctly explained.
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