51. Using many symbols makes it possible to put a large amount of information on a single map.

使用多种多样的符号可以在一张地图里放进大量的信息。

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.

无政府主义这个词描述的是一堆理论和态度，它们的主要共同点在于相信政府是有害的，没有必要的。

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.

恐怕没有谁对大多数美国人的日常生活影响能超过汽车生产的先驱亨利·福特。

54. The use of well-chosen nonsense words makes possible the testing of many basic hypotheses in the field of language learning.

使用精心挑选的无意义词汇，可以检验语言学科里许多基本的假定。

55. The history of painting is a fascinating chain of events that probably began with the very first pictures ever made.

优化历史是由一连串的迷人事件组成，其源头大概可以上溯到最早的图画。

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，串成一条项链，就能卖到比单独售出好得多的价钱。

57. During the eighteenth century，Little Turtle was chief of the Miami tribe whose territory became what is now Indiana and Ohio.

十八世纪时，“小乌龟”是迈阿密部落的酋长，该部落的地盘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.

在竹子的近七百个品种中，有的全长成还不到一英尺，有的却能在二十四小时内长出三英尺。

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.

谨慎的航海员在出航前，会研究航向，记录的灯塔的位置，以便对各种可能出现的情况做到有备无患。

60. Of all the economically important plants，palms have been the least studied.

在所有的经济作物中，棕榈树得到的研究最少。

61. Buyers and sellers should be aware of new developments in technology can and does affect marketing activities.  

购买者和销售者都应该留意技术的新发展，原因很简单，因为技术能够并且已经影响着营销活动。

62. The application of electronic controls made possible by the microprocessor and computer storage have multiplied the uses of the modern typewriter.

电脑储存和由于电子微处理机得以实现的电控运用成倍的增加了现代打字机的功能。

63. The human skeleton consists of more than two hundred bones bound together by tough and relatively inelastic connective tissues called ligaments.

人类骨骼有二百多块骨头组成，有些骨头是由坚韧而相对缺乏弹性的，被称为韧带的结蒂组连在一起。

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.

珍珠的色泽受到作为其母体牡蛎种类及牡蛎生活水域的深度，温度和含盐度的制约。

65. Although mockingbirds superbly mimic the songs and calls of many birds，they can nonetheless be quickly identified as mockingbirds by certain aural clues.

尽管模仿鸟学很多种鸟的鸣叫声惟妙惟肖，但人类还是能够依其声音上的线索很快识别它们。

66. Not only can walking fish live out of water，but they can also travel short distances over land.

鲇鱼不仅可以离开水存活，还可以在岸上短距离移动。

67. Scientists do not know why dinosaurs became extinct，but some theories postulate that changers in geography，climate，and sea levels were responsible.

科学家不知道恐龙为何绝种了，但是一些理论推断是地理，气候和海平面的变化造成的。

68. The science of horticulture，in which the primary concerns are maximum yield and superior quality，utilizes information derived from other sciences.

主要目的在于丰富和优质的农艺学利用了其他科学的知识。

69. Snow aids farmers by keeping heart in the lower ground levels，thereby saving the seeds from freezing.

雪对农民是一种帮助，因为它保持地层土壤的温度，使种子不致冻死。

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.

当代文学作品中的英雄本色虽各有千秋，但其昭世功力却是恒古不变的。

71. People in prehistoric times created paints by grinding materials such as plants and clay into power and then adding water.

史前的人们制造颜料是将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.

黄菊花通常令人生厌，它挤走不那么顽强的植物，并找来很多害虫。

73. Starting around 7000 B.C.，and for the next four thousand years，much of the Northern Hemisphere experienced temperatures warmer than at present.

大约从公元前七千年开始，在四千年当中，北半球的温度比现在高。

74. When Henry Ford first sought financial backing for making cars，the very notion of farmers and clerks owning automobiles was considered ridiculous.

当亨利·福特最初制造汽车为寻求资金支持时，农民和一般职员也能拥有汽车的想法被认为是可笑的。

75. Though once quite large，the population of the bald eagle across North America has drastically declined in the past forty years.

北美秃头鹰的数量一度很多，但在近四十年中全北美的秃头鹰数量急剧下降。