交变磁场的特点是.（ ）。

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SQL Error: select id,body,options from ***_enewstiku where sourceid='25690' and id < 2159440 and id<>'2159440' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku where sourceid='25690' and id > 2159440 and id<>'2159440' order by id asc limit 5