在文件系统中，文件的不同物理结构有不同的优缺点。在下列文件的物理结构中(15)结构不具有直接读写文件任意一个记录的能力。

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(75)is　a　clickable　string　or　graphic　that　points　to　another　Web　page　or　document。

A　typical　application　of　this(71)is　ADSL.　It　is　emerging　as　the　technology　for　home-and　small-office　Internet　connectivity.　It　provides　either　1.5　Mb/s～8　Mb/s　from　the　network　to　the　user　and　64　Kb/s～512　Kb/s　in　the　reverse　direction　depending　on　the　distance,　12　000　or　18　000　feet..　The　different　speed　for　each　direction　gives　it　the(72)label.　ADSL　is　designed　to　take　advantage　of　the　fact　that　video-on　demand,　telecommuting,　and　Internet　access　traffic　are　inherently　asymmetrical.　The　user(73)a　brief　message　up　to　the　network　and　receives　a　ton　of　data　coming　back,　either　a　movie　or　a　piece　of　data　download.　Under　such　a　scenario,　low-speed　traffic　to　the　network　is　just　fine.　ADSL　delivers　high(74)where　you　need　it　and　only　uses　a　single　copper　pair.　Through　multiplexing,　it　also　reserves(75)of　the　bandwidth　for　POTS.

(73)is　the　sending　and　receiving　of　the　message　by　computer.　It　is　a　fast,　　low-cost　way　of　communicating　worldwide。

(74)is　a　complex　consisting　of　two　or　more　connected　computing　units,　it　is　used　for　the　purpose　of　data　communication　and　resource　sharing。

Files　can　be　lost　or　destroyed　accidentally.　Keep(72)copies　of　all　data　on　removable　storage　media。

One　solution　to　major　security　problems　is(71),　which　are　frequently　installed　to　fix　known　security　holes。