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  3. DEFORESTATION IN NORTH AMERICA 1 The land area of the United States and Canada is just over 4.8 billion acres. When large numb

DEFORESTATION IN NORTH AMERICA 1 The land area of the United States and Canada is just over 4.8 billion acres. When large numb

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问题              DEFORESTATION IN NORTH AMERICA
1   The land area of the United States and Canada is just over 4.8 billion acres. When large numbers of Europeans began to arrive in the eighteenth century, almost one-third of that area was covered with old-growth forests. In the eastern half of the continent, nearly 90 percent of the land was thick with forests of elm, ash, beech, maple, oak, and hickory. By the end of the nineteenth century, after several decades of intensive deforestation, only half of the original forests remained.
2   During the first two centuries of European colonization, settlement was concentrated along the East Coast, having almost no effect on the vast forests covering the continent. Then, in the first half of the nineteenth century, agriculture expanded and settlers began to move westward in search of land for new farms. Land for agriculture came almost exclusively from clearing forests. The demand for farmland and timber continued to soar, and by 1850, more than 100 million acres of old-growth forest had been cut or burned off in the Northeast, the Southeast, the Great Lakes region, and along the St. Lawrence River.
3   Along with agriculture, industrialization was a major cause of deforestation. The Industrial Revolution was fueled by North America’s abundance of wood, as iron makers relied on charcoal, or charred wood, to fire their furnaces. Hardwoods such as oak produced the best charcoal, which charcoal burners made by slowly burning logs in kilns until they were reduced to concentrated carbon. It took eight tons of wood to make two tons of charcoal to smelt one ton of iron. Thus, the toll on the forests was high, as countless acres were cut to feed the furnaces of the iron industry.
4   The transportation technology of the Industrial Revolution contributed greatly to deforestation. The river steamboats that came into operation after 1830 had a voracious appetite for wood. To keep their wheels turning, steamboats typically took on fuel twice a day. The wood was supplied by thousands of"wood hawks" along the banks of the Ohio and Mississippi with stacks of cut firewood. Annual consumption of wood on riverboats continued to increase until 1865. Consequently, river valleys that had the heaviest traffic were stripped of their forests.
5   After 1860, immigration and westward expansion surged, and railroads swept over the continent. Clean-burning hardwood was the preferred fuel of the "iron horses," which required the cutting of 215,000 acres of woodland to stay in operation for one year. Not only did wood fuel the steam engines, but enormous amounts of oak and locust also went into the manufacture of railcars, ties, fencing, bridges, and telegraph poles. Railroads in the United States and Canada stretched from coast to coast by 1885, and each additional mile of railroad meant at least two more miles of fencing and 2,500 ties.
6   Other major consumers of forest products included ordinary homeowners. More than four out of five of the houses constructed in the early nineteenth century-from log cabins to clapboard cottages-were built mainly of wood and roofed with wooden shingles. All were filled with wooden furniture. Two-thirds of all households in North America were heated by open, wood-burning fireplaces, and it took between 10 and 20 acres of forest to keep a single fireplace burning for one year.
7   Throughout the century, the timber industry continued to supply the single most valuable raw material for a rapidly expanding population. Between 1840 and 1860, the annual production of lumber rose from 1.6 million to 8 billion board feet. This increase was made possible by the widespread application of steam power. Wood-fueled steam engines powered the sawmills, moved and barked the logs, and finished the boards. Railroad lines were now built right into the forests so that felled logs could be shipped directly to market. These innovations had their greatest impact in the Great Lakes region. By 1890 the technology of the timber industry had triumphed over the natural abundance of the forests, and woodlands that had once seemed endless were now depleted.
The word they in paragraph 3 refers to
选项 A、furnaces
B、logs
C、kilns
D、forests
答案B
解析 The referent of they is something that was reduced to concentrated carbon. The previous clause states that charcoal burners made charcoal by burning logs. Logic tells you that they refers to logs.  
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