图所示电路中，所有电阻均为3Ω,a,b两端的等效电阻为（）。 

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SQL Error: select id,body,options from ***_enewstiku where sourceid='32670' and id < 2598646 and id<>'2598646' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku where sourceid='32670' and id > 2598646 and id<>'2598646' order by id asc limit 5