主变压器的作用是（  ）。  

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69483' and id < 4500466 and id<>'4500466' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69483' and id > 4500466 and id<>'4500466' order by id asc limit 5