单选题 One thing that distinguishes the online world from the real one is that it is very easy to find things. To find a copy of The Economist in print, one has to go to a news-stand, which may or may not carry it. Finding it online, though, is a different proposition. Just go to Google, type in "economist" and you will be instantly directed to economist.com. Though it is difficult to remember now, this was not always the case. Indeed, until Google, now the world's most popular search engine, came on to the scene in September 1998, it was not the case at all. As in the physical world, searching online was a hit-or-miss affair.
Google was vastly better than anything that had come before: so much better, in fact, that it changed the way many people use the web. Almost overnight, it made the web far more useful, particularly for nonspecialist users, many of whom now regard Google as the internet's front door. The recent fuss over Google's stock market flotation obscures its far wider social significance: few technologies, after all, are so influential that their names become used as verbs.
Google began in 1998 as an academic research project by Sergey Brin and Lawrence Page, who were then graduate students at Stanford University in Palo Alto, California. It was not the first search engine, of course. Existing search engines were able to scan or "crawl" a large portion of the web, build an index, and then find pages that matched particular words. But they were less good at presenting those pages, which might number in the hundreds of thousands, in a useful way.
Mr Brin's and Mr Page's accomplishment was to devise a way to sort the results by determining which pages were likely to be most relevant. They did so using a mathematical recipe, or algorithm, called PageRank. This algorithm is at the heart of Google's success, distinguishing it from all previous search engines and accounting for its apparently magical ability to find the most useful web pages.
Untangling the web
PageRank works by analysing the structure of the web itself. Each of its billions of pages can link to other pages, and can also, in turn, be linked to. Mr Brin and Mr Page reasoned that if a page was linked to many other pages, it was likely to be important. Furthermore, if the pages that linked to a page were important, then that page was even more likely to be important. There is, of course, an inherent circularity to this formula--the importance of one page depends on the importance of pages that link to it, the importance of which depends in turn on the importance of pages that link to them. But using some mathematical tricks, this circularity can be resolved, and each page can be given a score that reflects its importance.
The simplest way to calculate the score for each page is to perform a repeating or "iterative" calculation (see article). To start with, all pages are given the same score. Then each link from one page to another is counted as a "vote" for the destination page. Each page's score is recalculated by adding up the contribution from each incoming link, which is simply the score of the linking page divided by the number of outgoing links on that page. (Each page's score is thus shared out among the pages it links to.)
Once all the scores have been recalculated, the process is repeated using the new scores, until the scores settle down and stop changing (in mathematical jargon, the calculation "converges"). The final scores can then be used to rank search results: pages that match a particular-set of search terms are displayed in order of descending score, so that the page deemed most important appears at the top of the list.

单选题 We can infer from the 1st paragragh that by "hit-or-miss" it is meant ______.

单选题 "Though it is difficult to remember now, this was not always the case." In the 1st paragragh, this sentence suggests that ______.

单选题 The most important factor in Google's success is ______.

单选题 "But they were less good at presenting those pages, which might number in the hundreds of thousands, in a useful way." This sentence in the 3rd paragraph tells us that ______.

单选题 Which of the following is NOT true?