电路如图3所示，电路中电压U等于（   ）。 

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69473' and id < 4501201 and id<>'4501201' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69473' and id > 4501201 and id<>'4501201' order by id asc limit 5