电路如图所示，电阻R可变，为使R获得最大功率，R应为（   ）。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69474' and id < 4499646 and id<>'4499646' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69474' and id > 4499646 and id<>'4499646' order by id asc limit 5