若异步电动机在频率为50Hz时额定转速为960r/min，则电机极数为（   ）。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69401' and id < 4493282 and id<>'4493282' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69401' and id > 4493282 and id<>'4493282' order by id asc limit 5