50. The ancient Hopewell people of North America probably cultivated corn and other crops, but hunting and gathering were still of critical importance in their economy.
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.
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.
54. The use of well-chosen nonsense words makes possible the testing of many basic hypotheses in the field of language learning.
55. The history of painting is a fascinating chain of events that probably began with the very first pictures ever made.
56. Perfectly matched pearls, strung into a necklace, bring a far higher price than the same pearls told individually.
57. During the eighteenth century, Little Turtle was chief of the Miami tribe whose territory became what is now Indiana and Ohio.
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.
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.
60. Of all the economically important plants, palms have been the least studied.
61. Buyers and sellers should be aware of new developments in technology can and does affect marketing activities.
62. The application of electronic controls made possible by the microprocessor and computer storage have multiplied the uses of the modern typewriter.
63. The human skeleton consists of more than two hundred bones bond together by tough and relatively inelastic connective tissues called ligaments.
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.
65. Although mockingbirds superbly mimic the songs and calls of many birds, they can nonetheless be quickly identified as mockingbirds by certain aural clues.
66. Not only can walking fish live out of water, but they can also travel short distances over land.
67. Scientists do not know why dinosaurs became extinct, but some theories postulate that changers in geography, climate, and sea levels were responsible.
68. The science of horticulture, in which the primary concerns are maximum yield and superior quality, utilizes inFORMation derived from other sciences.
69. Snow aids farmers by keeping heart in the lower ground levels, thereby saving the seeds from freezing.
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.
71. People in prehistoric times created paints by grinding materials such as plants and clay into power and then adding water.
72. Often very annoying weeds, goldenrods crowd out less hardy plants and act as hosts to many insect pests.
73. Starting around 7000 B.C., and for the next four thousand years, much of the Northern Hemisphere experienced temperatures warmer than at present.
74. When Henry Ford first sought financial backing for making cars, the very notion of farmers and clerks owning automobiles was considered ridiculous.
75. Though once quite large, the population of the bald eagle across North America has drastically declined in the past forty years.
76. The beaver chews down trees to get food and material with which to build its home.
77. Poodles were once used as retrievers in duck hunting, but the American Kennel Club does not consider them sporting dogs because they are now primarily kept as pets.
78. As a result of what is now know in physics and chemistry, scientists have been able to make important discoveries in biology and medicine.
79. The practice of making excellent films based on rather obscure novels has been going on so long in the United States as to constitute a tradition.
80. Since the consumer considers the best fruit to be that which is the most attractive, the grower must provide products that satisfy the discerning eye.
81. Television the most pervasive and persuasive of modern technologies, marked by rapid change and growth, is moving into a new era, an era of extraordinary sophistication and versatility, which promises to reshape our lives and our world.
84. William Rainey Harper lured him to the new university of Chicago, where he remained officially for exactly a generation and where his students in advanced composition found him terrifyingly frigid in the classroom but sympathetic and understanding in their personal conferences.
84.他的教书生涯始于麻省理工学院，但是William Rainey Harper把他吸引到了新成立的芝加哥大学。他在那里正式任职长达整整一代人的时间。他的高级作文课上的学生觉得他在课上古板得可怕，但私下交流却富有同情和理解。
85. The sloth pays such little attention to its personal hygiene that green algae grow on its coarse hair and communities of a parasitic moth live in the depths of its coat producing caterpillars which graze on its mouldy hair. Its muscles are such that it is quits incapable of moving at a speed of over a kilometer an hour even over the shortest distances and the swiftest movement it can make is a sweep of its hooked arm.
86. Artificial flowers are used for scientific as well as for decorative purposes. They are made from a variety of materials, such as way and glass, so skillfully that they can scarcely be distinguished from natural flowers.
87. Three years of research at an abandoned coal mine in Argonne, Illinois, have resulted in findings that scientists believe can help reclaim thousands of mine disposal sites that scar the coal-rich regions of the United States.
88. When the persuading and the planning for the western railroads had finally been completed, the really challenging task remained: the dangerous, sweaty, backbreaking, brawling business of actually building the lines.
89. Because of the space crunch, the Art Museum has become increasingly cautious in considering acquisitions and donations of art, in some cases passing up opportunities to strengthen is collections.
90. The United States Constitution requires that President be a natural-born citizen, thirty-five years of age or older, who has lived in the United States for a minimum of fourteen years.
91. Arid regions in the southwestern United States have become increasingly inviting playgrounds for the growing number of recreation seekers who own vehicles such as motorcycles or powered trail bikes and indulge in hill-climbing contests or in caving new trails in the desert.
92. Stone does decay, and so tools of long ago have remained when even the bones of the man who made them have disappeared without trace.
93. Insects would make it impossible for us to live in the world; they would devour all our crops and kill our flocks and herds, if it were not for the protection we get from insect-eating animals.
94. It is true that during their explorations they often faced difficulties and dangers of the most perilous nature, equipped in a manner which would make a modern climber shudder at the thought, but they did not go out of their way to court such excitement.
95. There is only one difference between an old man and a young one: the young man has a glorious future before him and old one has a splendid future behind him: and maybe that is where the rub is.
96. I find young people exciting. They have an air of freedom, and they have not a dreary commitment to mean ambitions or love comfort. They are not anxious social climbers, and they have no devotion to material things.
97. I am always amazed when I hear people saying that sport creates goodwill between the nations, and that if only the common peoples of the world could meet one another at football or cricket, they would have no inclination to meet on the battlefield.
98. It is impossible to say simply for the fun and exercise: as soon as the question of prestige arises, as soon as you feel that you and some larger unit will be disgraced if you lose, the most savage combative instincts are around.
99. It has been found that certain bats emit squeaks and by receiving the echoes, they can locate and steer clear of obstacles------or locate flying insects on which they feed. This echo-location in bats is often compared with radar, the principle of which is similar.
100. As the time and cost of making a clip drop to a few days and a few hundred dollars, engineers may soon be free to let their imaginations soar without being penalized by expensive failure.