宁夏中部风沙区的沙质荒漠化在历史时期，尤其是本世纪50年代以来，具有明显扩展的趋势。

There is an obvious developing tendency of sandy desertification in the blowing sand region of middle Ningxia in historical time, especially since 1950's.

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无疑自然因素在历史时期疏勒河流域河湖水系变迁中充当了重要角色，社会因素各个时段的作用却不尽相同。

Undoubtedly natural factors play an important role in water network changes of Shule River Basin in historical period, but the effect of social factors in various periods is different.

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除了观赏展品，游客还可以玩电脑模拟游戏，想象自己生活在不同的历史时期，或者漫步在热带雨林中。

As well as looking at exhibits, visitors can play with computer simulations and imagine themselves living at a different time in history or walking through a rainforest.

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So I started to talk to the VCs and asking for $10 million at a very, very difficult time in Internet history.

就在互联网历史上最艰难的时期，我开始跟风险投资商接洽，向他们要1000万美元。

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We've probably had four major periods in American history of there's one other picture I wanted to put up.

在美国历史上,我们大约经历了四个时期,在我想贴的另一张图片上

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There's a reason why the first attempt to even write about international law comes at this period.

这也就是为什么在这个时期,会有人在历史上第一次尝试撰写国际法

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