51. Using many symbols makes it possible to put a large amount of information on a single map. 使用多種多樣的符號(hào)可以在一張地圖里放進(jìn)大量的信息。

　　52. Anarchism is a term describing a cluster of doctrines and attitudes whose principal uniting feature is the belief that government is both harmful and unnecessary. 無政府主義這個(gè)詞描述的是一堆理論和態(tài)度，它們的主要共同點(diǎn)在于相信政府是有害的，沒有必要的。

　　53. Probably no man had more effect on the daily lives of most people in the Untied States than did Henry Ford a pioneer in automobile production. 恐怕沒有誰對(duì)大多數(shù)美國人的日常生活影響能超過汽車生產(chǎn)的先驅(qū)亨利.福特。

　　54. The use of well-chosen nonsense words makes possible the testing of many basic hypotheses in the field of language learning. 使用精心挑選的無意義詞匯，可以檢驗(yàn)語言學(xué)科里許多基本的假定。

　　55. The history of painting is a fascinating chain of events that probably began with the very first pictures ever made. 優(yōu)化歷史是由一連串的迷人事件組成，其源頭大概可以上溯到最早的圖畫。

　　56. Perfectly matched pearls, strung into a necklace, bring a far higher price than the same pearls told individually. 相互般配的珍珠，串成一條項(xiàng)鏈，就能賣到比單獨(dú)售出好得多的價(jià)錢。

　　57. During the eighteenth century, Little Turtle was chief of the Miami tribe whose territory became what is now Indiana and Ohio. 十八世紀(jì)時(shí)，“小烏龜”是邁阿密部落的酋長，該部落的地盤就是今天的印第安那州和俄亥俄州。

　　58. Among almost seven hundred species of bamboo, some are fully grown at less than a foot high, while others can grow three feet in twenty-four hours. 在竹子的近七百個(gè)品種中，有的全長成還不到一英尺，有的卻能在二十四小時(shí)內(nèi)長出三英尺。

　　59. Before staring on a sea voyage, prudent navigators learn the sea charts, study the sailing directions, and memorize lighthouse locations to prepare themselves for any conditions they might encounter. 謹(jǐn)慎的航海員在出航前，會(huì)研究航向，記錄的燈塔的位置，以便對(duì)各種可能出現(xiàn)的情況做到有備無患。

　　60. Of all the economically important plants, palms have been the least studied. 在所有的經(jīng)濟(jì)作物中，棕櫚樹得到的研究最少。

61. Buyers and sellers should be aware of new developments in technology can and does affect marketing activities. 購買者和銷售者都應(yīng)該留意技術(shù)的新發(fā)展，原因很簡單，因?yàn)榧夹g(shù)能夠并且已經(jīng)影響著營銷活動(dòng)。

　　62. The application of electronic controls made possible by the microprocessor and computer storage have multiplied the uses of the modern typewriter. 電腦儲(chǔ)存和由于電子微處理機(jī)得以實(shí)現(xiàn)的電控運(yùn)用成倍的增加了現(xiàn)代打字機(jī)的功能。

　　63. The human skeleton consists of more than two hundred bones bound together by tough and relatively inelastic connective tissues called ligaments. 人類骨骼有二百多塊骨頭組成，住些骨頭石油堅(jiān)韌而相對(duì)缺乏彈性的，被稱為韌帶的結(jié)蒂組連在一起。

　　64. The pigmentation of a pearl is influenced by the type of oyster in which it develops and by the depth, temperature, and the salt content of the water in which the oyster lives. 珍珠的色澤受到作為其母體牡蠣種類及牡蠣生活水域的深度，溫度和含鹽度的制約。

　　65. Although mockingbirds superbly mimic the songs and calls of many birds, they can nonetheless be quickly identified as mockingbirds by certain aural clues. 盡管模仿鳥學(xué)很多種鳥的鳴叫聲惟妙惟肖，但人類還是能夠依其聲音上的線索很快識(shí)別它們。

　　66. Not only can walking fish live out of water, but they can also travel short distances over land. 鲇魚不僅可以離開水存活，還可以在岸上短距離移動(dòng)。

　　67. Scientists do not know why dinosaurs became extinct, but some theories postulate that changers in geography, climate, and sea levels were responsible. 科學(xué)家不知道恐龍為何絕種了，但是一些理論推斷是地理，氣候和海平面的變化造成的。

　　68. The science of horticulture, in which the primary concerns are maximum yield and superior quality, utilizes information derived from other sciences. 主要目的在于豐富和優(yōu)質(zhì)的農(nóng)藝學(xué)利用了其他科學(xué)的知識(shí)。

　　69. Snow aids farmers by keeping heart in the lower ground levels, thereby saving the seeds from freezing. 雪對(duì)農(nóng)民是一種幫助，因?yàn)樗�保持地層土壤的溫度，使種子不致凍死。

　　70. Even though the precise qualities of hero in literary words may vary over time, the basic exemplary function of the hero seems to remain constant. 當(dāng)代文學(xué)作品中的英雄本色雖各有千秋，但其昭世功力卻是恒古不變的。

　　71. People in prehistoric times created paints by grinding materials such as plants and clay into power and then adding water. 史前的人們制造顏料是將植物和泥土等原料磨成粉末，然后加水。

　　72. Often very annoying weeds, goldenrods crowd out less hardy plants and act as hosts to many insect pests. 黃菊花通常令人生厭，它擠走不那么頑強(qiáng)的植物，并找來很多害蟲。

　　73. Starting around 7000 B.C., and for the next four thousand years, much of the Northern Hemisphere experienced temperatures warmer than at present. 大約從公元前七千年開始，在四千年當(dāng)中，北半球的溫度比現(xiàn)在高。

　　74. When Henry Ford first sought financial backing for making cars, the very notion of farmers and clerks owning automobiles was considered ridiculous. 當(dāng)亨利.福特最初制造汽車為尋求資金支持時(shí)，農(nóng)民和一般職員也能擁有汽車的想法被認(rèn)為是可笑的。

　　75. Though once quite large, the population of the bald eagle across North America has drastically declined in the past forty years. 北美禿頭鷹的數(shù)量一度很多，但在近四十年中全北美的禿頭鷹數(shù)量急劇下降。