在数据库系统中，若某基本层次联系的集合满足以下两个条件：有且仅有一个结点无双亲；其它结点有且仅有一个双亲，则该集合称为(20)。

(69)

(70)

(67)

(68)

There　is　no　question　that　computer　applications　such　as(71),　electronic　mail,　and(72)publishing　have　changed　the　way　people　work.　These　computer　applications　have　enhanced　users　capacity　for　communication　and　have　improved　their　productivity.　The　success　of　these　applications　has　prompted　both　vendors　and　researchers　to　continue　to　seek　new　ways　to　further　advance　the　information　technology　revolution.　Enter　the　latest　innovation:　networked(73)systems.　They　convey　information　in　multiple(74):　text,　graphics,　video,　audio,(75),　computer　　simulations,　and　so　forth.　Advocates　of　these　systems　argue　that　they　are　the　next　logical　step　in　human　communication;　however,　modifying　our　current　networks　to　accommodate　the　variety　of　media　will　present　significant　technical　difficulties.

(64)

(65)

(63)

(62)

(60)