下图所示电路中，若U2=U-U1，则方框中接入的元件为（   ）。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69471' and id < 4499915 and id<>'4499915' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69471' and id > 4499915 and id<>'4499915' order by id asc limit 5