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nal body surfaces, on lining membranes(内膜) and inside tubes and organs that are connected with the outside—such as the mouth, throat, nose, and large intestine(肠)—both harmless and pathogenic(病原的)microbes(微生物)live in large numbers. 47 . If these natural defenses are sound, a person may conceal many microbes without ill effect. However, if his resistance becomes lowered, an infection can result.
　　Microbes are spread about through an almost infinite variety of means. 48 . The more crowded the conditions, the greater the chances of epidemics(流行病) breaking out. 49 .
　　Although microbes cannot fly, the dust of the air is alive with them. In close quarters the atmosphere is further polluted by bacteria-loaded tiny drops discharged by coughing and even normal conversation. Pathogenic microbes are also spread by rats, mice, flies and mosquitoes. 50 . In slum areas there is often a combination of crowded conditions and poor sanitation(环境卫生). In addition, many of the inhabitants have lowered resistance because of shortage of food and other undesirable unhealthy practices. As a result, epidemics are likely to begin in these districts.
　　All civilized societies establish and enforce measures designed to protect the health of their populations. Most of these practices are concerned with preventing the spread of the pathogenic organisms
　　A The atmosphere is a carrier of microorganisms
　　B One exception is the interior of normal body tissue
　　C Microbial growth is further helped by the widespread presence of dirt and the lack of sunlight
　　D The body has natural defenses against these organisms
　　E The body has no defenses against these organisms
　　F The simplest way is by person-to-person contact

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　　Taking a Nap During the Day
　　Medical experts say most Americans do not get 51 sleep. They say more Americans need to rest for a short period in the middle of the day. They are advising people to sleep lightly before 52 with other activities.
　　One study earlier this year found that persons who sleep for a few minutes during the day were less 53 to die of heart disease. The study followed more than 2,300 Greek adults 54 about six years. Adults who rested for half an hour 55 three times a week had a 37 present lower risk of dying from heart disease than those who did not nap.
　　Study organizers said the strongest evidence was in working men. They said naps might improve 56 by mitigating tension caused by work.
　　Some European and Latin American businesses have supported the 57 of napping for many years. They urge people to 58 work, go home and have a nap before returning. In the United States, some companies let workers rest briefly in their offices. They believe this reduces 59 and accidents, and 60 increases the amount of work a person can do.
　　Sleep experts say it is likely that people make more mistakes at work than at other times. They say people should not carry out important duties when they feel 61 And they say the best thing to do is to take a nap. About twenty minutes of rest is 62 you need. Experts say this provides extra energy and can increase your effectiveness 63 the end of the day. But experts warn that a nap 64 last no more than twenty to thirty minutes. A longer nap will put the body into deep sleep and waking up will be 65 .
　　51. A sweet B sound C bad D enough
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