声波发射时，能在两个结构的界面产生反射 必须具备的因素是（     ）

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SQL Error: select id,body,options from ***_enewstiku where sourceid='25734' and id < 2163174 and id<>'2163174' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku where sourceid='25734' and id > 2163174 and id<>'2163174' order by id asc limit 5