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  3. If you have lots of unused storage space on your hard disk, then why not share it with others on the Internet? The benefit could

If you have lots of unused storage space on your hard disk, then why not share it with others on the Internet? The benefit could

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问题     If you have lots of unused storage space on your hard disk, then why not share it with others on the Internet? The benefit could be distributed storage for your own files, making them available any time via the web, even if you are nowhere near your computer—indeed, even if your computer is switched off. That idea is what a Zurich-based firm called Caleido is aiming to provide, with a free online storage service known as Wuala that was recently introduced to the public.
    【R1】______In particular, its developers, Dominik Grolimund and Luzius Meisser, have used a clever mathematical trick to compensate for the fact that the participating computers will come and go from the internet in an unpredictable way.
    【R2】______Copying costs participants both storage space and bandwidth.
    Yet there have to be enough copies to ensure that there is at least one available most of the time. If, for example, each computer is online 25% of the time, then a quick calculation shows that you would have to copy each file to 100 different computers to ensure that 999,999 times out of a million there is at least one copy available when a user looks.
    【R3】______Instead, Mr. Grolimund and Mr. Meisser plan to break each file into chunks, which can be scattered liberally around the hard disks of participating computers, and then to use a mathematical trick to reconstruct the original file from those chunks.
    【R4】______The first step is to convert the file(which is, regardless of what it represents, simply a long string of ones and zeros)into a mathematical function called a polynomial. This is done by splitting it into 100 fragments, which are smaller binary numbers. It is these numbers that are used to define the polynomial. All you have to do is select a suitable number of points from along the polynomial and convert their values into the appropriate mathematical constructs. Scatter these around the host computers and, when someone wants to look at the file, he need recover only 100 of them to have enough data to reconstruct the file from scratch. To have 100 points available 999,999 times out of a million it turns out that you need to scatter a total of 600 of them around. That is an amount of data equivalent to six versions of the original file. Moreover, the system needs the computers linked to it to be available for only 17% of the time, rather than 25% , for this to apply.
    【R5】______Most commercial online-storage services use centralized servers. Although these are generally reliable, they do sometimes fail. And when they do, the results are embarrassing.
    Though some people may feel sick about scattering their data over hundreds of other computers(even though it will be encrypted), or storing unknown file fragments on their own, Mr Grolimund is sure that Caleido has learnt from other "peer-to-peer" file-sharing systems, and that Wuala is built to handle concerns about the illegal distribution of copyrighted or " inappropriate" content.
[A]This trick, known as Reed-Solomon error correction, is employed routinely to interpret the data on DVDs, but it has not been used before in the volatile world of private computers on the Internet.
[B]One of the characteristics of a polynomial is that a few numbers can nail it down precisely. If a simple polynomial is plotted out on a graph it forms a line.
[C]Online storage is a growing market, especially for backing up data, where reliability is a big concern.
[D]The challenge is how to minimize the number of copies of the same file that have to be distributed.
[E]It has been proved that for online data storage, Wuala is as good to give as it is to receive.
[F]Though the idea underlying it is simple, Wuala requires some nifty technology to make its distributed system work reliably.
[G]But copying every file a hundred times is hugely inefficient.
【R4】
选项
答案A
解析 前文已经提到了开发Wuala时遇到的拷贝问题以及两位开发者针对这一问题使用了一个数学技巧，而本段中从数学的角度出发对此进行了较为详细的说明，也就是如何对文件加以处理以减小拷贝数量，第一步是把文件转换成多项式，然后在多项式上选择适当数量的点，接下来将它们的值转换成恰当的数学形式，最后将这些点散布在电脑主机上。可见本段开头应该提出了一个概念，然后进行详细的解释，据此判断正确答案为[A]“这种技巧称为里德一所罗门错误校验……”，[A]很好地承接了上一段最后一句话中的“用数学技巧将这些块重建得到原始文档。”
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