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。

76. The beaver chews down trees to get food and material with which to build its home. 水獭啃倒树木，以便取食物并获得造窝的材料。

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. 长卷毛狗曾被用作猎鸭时叼回猎物的猎犬，但是美国Kennel Club却不承认它们为猎犬，因为它们现在大多数作为宠物饲养。

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. 物理学和化学的一个成果是使得科学家们能在生物学和医学上获得重大发现。

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. 根据默默无闻的小说制作优秀影片在美国由来已久，已经成为传统。

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. 因为顾客认为最好的水果应该看起来也是最漂亮的，所以种植者必须提供能满足挑剔眼光的产品。