前置模拟低通滤波器可以起到抑制高频无用信号的作用（ ）。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69493' and id < 4501579 and id<>'4501579' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69493' and id > 4501579 and id<>'4501579' order by id asc limit 5