托福考试易错题（2019/3/7）

NorthAmerican Grasslands
In NorthAmerica, native grasslands occur primarily in the Great Plains in the middle of the continent. The NorthAmerican prairie biome is one of the most extensive grasslands in the world, extending from the edge of the Rocky Mountains in the west to the deciduous forest in the east, and from northern Mexico in the south toCanada in the north.Average annual rainfall ranges from about 40 cm (16 inches) in the west to 80 cm (31 inches) in the east.Average annual temperatures range between 10 degrees and 20 degreesCelsius (50 to 68 degrees Fahrenheit). In the moist regions of the NorthAmerican grasslands, especially in the northern Great Plains, rainfall is distinctly seasonal, and temperatures can vary widely from very hot in summer to bitter cold in winter.
One hundred years ago, the Great Plains grasslands were one vast, unbroken prairie.
Much of the prairie is now farmland, the most productive agricultural region in the world, dominated by monocultures of cereal grains.
Wheat, barley, soybeans, corn, and sunflowers occupy the land that was once prairie.
In areas given over to grazing lands for cattle and sheep, virtually all the major native grasses have been replaced by alien species.

An important feature of the northern Great Plains grasslands is the presence of millions of glacial depressions that are now small ponds known as prairie potholes. They were formed during the most recent IceAge, when streams flowed in tunnels beneath glacially formed sandy ridges. When the IceAge ended around 12,000 years ago, the retreating glaciers created about 25 million depressions across a 300,000-square-mile landscape--about 83 potholes per square mile.As the ice blocks melted, much of the water was left behind, forming wetlands ranging in size from a tenth of an acre to several acres. The wetlands were soon surrounded by fluttering waves of grasses: shortgrass, mixed grass, and tallgrass.
Today these small wetlands still cover the prairies, although much of the landscape including both native grasses and potholes has been transformed to cropland and grassland for grazing. What does remain of the wetlands, however, still serves as an important breeding area for more than 300 bird species, including large numbers of migrating shorebirds and waterfowl. The potholes fill up with water during spring rains and usually dry out by late summer.Every spring, birds arrive in great numbers--northern pintails, mallards, coots, and pied-billed grebes--4 to 6 million strong, to mate in the seasonal wetlands that dot portions of Minnesota, Iowa, North and SouthDakota, Montana,Alberta, Saskatchewan, and ManitobA、Prairie pothole country produces half of NorthAmerica’s 35 to 40 million ducks and is renowned worldwide as a "duck factory".
Recently biologists have discovered that the prairie pothole region is potentially a vast carbon sink: a natural sponge that absorbs carbon dioxide emissions from cars, factories, and power plants.Carbon dioxide is the most common of all the pollutants acting as greenhouse gases that heat up the atmosphere. Fortunately, however, carbon dioxide is captured naturally and stored in trees, soil, and plants. Scientists have termed this "carbon sequestration". They have determined that prairie potholes hold an average of 2.5 tons of carbon per acre per year when not being farmeD、This means that if the entire pothole region in the United States andCanada were to stop
being farmed, the region would store about 400 million tons of carbon over 10 years--the equivalent of taking almost 4 million cars off the

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