Shell viruses wrap themselves around a host program and don't modify the original program．Shell programs are easy to write，which is why about half of viruses are of this type．

外壳型病毒将它们自己包裹在主程序的四周，对源程序不作修改。 外壳型病毒容易编写，这也正是约有一半病毒程序是这种类型的原因。

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A year and a half ago, USA Today published an interesting article as a kind of wrap-up of the four-day annual meeting of the American Psychological Association.

一年半前，《今日美国》报上登载了一篇有趣的文章作为美国心理协会为期四天的年会总结报告。

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Although each participant received an identical gift, half of the gifts were covered nicely in wrapping paper, while the other half were given without gift wrap.

虽然每一位与者都收到相同的礼物，一半的礼物用精美的包装纸包装好的，而另外的一半就没有包装。

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EU's president for the first half of 1999: wrap up Agenda 2000, including the budget, at a special summit in March.

ECONOMIST: Agenda 1999

Brawn, whose car is powered by a Mercedes engine, needed half a point to wrap up the title in Brazil, ahead of closest rivals Red Bull, and they picked up five.

BBC: Brawn win title in debut F1 year

Thirty-eight percent expect 76 to 99 percent of their systems will be Y2K-ready, while the remaining 6 percent of companies will wrap up testing on half to three-quarters of their systems by year's end.

CNN: 8 of 10 companies expect no major Y2K woes