当使用电子邮件访问POP3服务器时，(21)。

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The　Internet　is　a(66)network　of　networks　which　interconnects(67)ranging　from　the　simplest　personal　computers　to　the　most　sophisticated　mainframes　and　supercomputers.　This　network　of　networks　is　used　for　an　extraordinary　range　of　purposes　-　long　distance　collaborations,　retrieving　free　software　and　documents,(68)library　catalogs,(69)in　t6　supercomputers　-　the　list　of　current　Internet　activities　and(70)is　large　and　the　range　of　possible　future　applications　is　as　unlimited　as　our　imagination.

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Programmers　test　a　program　by　running　it　to　ensure　that　the(74)are　correct　and　that　the　program　produces　the　desired　outcome.　If　errors　do　occur,　the　programmer　must　make　the　appropriate　change　and　recheck　the　program　until　it　produces　the　correct　results.　This(75)is　called　testing　and　debugging.

Computer　hardware　engineers　research,　design,　develop,　and　test　computer　hardware　and　supervise　its　manufacture　and(72).　Hardware　refers　to　computer(73),　circuit　boards,　computer　systems,　and　related　equipment　such　as　keyboards,　modems,　and　printers.

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