第6部分：完形填空(第51～65题，每题1分，共15分)下面的短文有15处空白，请根据短文内容为每处空白确定1个最佳选项。

Germs(细菌) on Banknotes

        People in different countries use different types of money: yuan in China, pesos in Mexico, pounds in the United Kingdom, dollars in the United States, Australia and New Zealand. They may usedifferent currencies, but these countries, and probably all countries, still have one thing in ______ (51)(变化): germs on the banknotes.

        Scientists have been studying the germs on money for well over。100 years. At the turn of the 20th ______ (52), some researchers began to suspect that germs living on money could spread disease.Most studies of germy money have looked at the germs on the currency ______ (53) one country. In a new study, Frank Vriesekoop and other researchers compared the germ populations found on bills of different ______ (54).

        Vriesekoop is a microbiologist at the University of Ballarat in Australia. He led the study, which compared the germ populations found on money ______ (55) from 10 nations. The scientists studied l,280 banknotes in total; all came from places where people buy food, like supermarkets, street vendors and cafes, ______ (56) those businesses often rely on cash.

        Overall, the Australian dollars hosted the fewest live bacteria---no more than 10 per square centimeter. Chinese yuan had the ______(57)(变化)---about 100 per square centimeter. Most of the germs on money probably would not cause harm.What we call             "paper money" ______ (58)(变化)isn’t made from paper.

 The U.S. dollar, for example, is printed on fabric that is mostly cotton. Different countries may use different ______ (59) to print their money. Some of the currencies studied by Vriesekoop and his team, such as the American dollar, were made from cotton. Others were made from polymers.

         The three ______ (60) with the lowest numbers of bacteria were all printed on polymers. They included the Australian dollar, the New Zealand dollar and some Mexican pesos.

         The ______ (61) (变化)currencies were printed on fabric made mostly of cotton. Fewer germs lived on the polymer notes. This ______ (62)(变化) suggests that germs have a harder time staying alive on polymer surfaces. Scientists need to do more studies to understand ______ (63) (变化)germs live on money---and whether or not we need to be concerned. Vriesekoop is now starting a study that will ______ (64) the amounts of time bacteria can stay alive on different types of bills.Whatever Vriesekoop finds, the fact remains：Paper money ______ (65)(变化) germs. We should wash our hands after touching it; After all, you never know where your money's been. Or what's living on it.    

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