我国交流电的频率为50Hz，其周期为(    )秒。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69456' and id < 4498150 and id<>'4498150' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69456' and id > 4498150 and id<>'4498150' order by id asc limit 5