电力系统中的频率是由(    )决定。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69507' and id < 4502773 and id<>'4502773' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69507' and id > 4502773 and id<>'4502773' order by id asc limit 5