平均自由行程长度λ：粒子单位行程中碰撞次数。（   ）

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='68449' and id < 4418999 and id<>'4418999' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='68449' and id > 4418999 and id<>'4418999' order by id asc limit 5