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They can be exhausted, completely due to, , over-consumption, , except for hydel, power., , They pollute — the, environment by, emitting harmful, gases and also, contribute to global, warming., , They are commonly, used for industrial and, , commercial purposes., , Heavy expenditure is, involved in using and, maintaining these, , sources of energy., , They are used, extensively, at a higher, rate than the non, conventional sources., , They cannot be, , exhausted completely., , They are environmentfriendly, do not pollute, , the environment., , They used, , commonly, , are, used for, , household purposes., , Using these sources is, , less expensive., , They are not used as, extensively as, , conventional sources.