1. In North America, native grasslands occur primarily in the Great Plains in the middle of the continent. The North American prairie biome is one of the most extensive grasslands in the world, extending from the edge of the Rocky Mountain in the west to the deciduous forest in the east, and from northern Mexico in the south to Canada in the north. Average annual rainfall ranges from about 40cm (16 inches) in the west to 80cm (31 inches) in the east. Average annual temperatures rang between 10 degrees and 20 degrees Celsius (50 to 68 degrees Fahrenheit). In the moist regions of the North American grasslands, esp. in the northern Great Plains, rainfall is distinctly seasonal, and temperatures can vary widely from very hot in summer to bitter cold in winter.
2. 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, soybean, corn, and sunflowers occupy the land that was once prairie. In the areas given over to grazing lands for cattle and sheep, virtually all the major native grasses have been replaced by alien species.
3. 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 Ice Age, when streams flowed in tunnels beneath glacially formed sandy ridges. When the Ice Age 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 if an acre to several acres. The wetlands were soon surrounded by fluttering waves of grasses: shortgrass, mixed grass, and tallgrass.
1. All of the following statements are true of the northern Great Plains EXCEPT
A. Summer temperatures are very hot, while winter ones are very cold.
B. An unbroken prairie now extends from the western mountains to the eastern forest.
C. The original vegetation consisted of shortgrass, mixed grass, and tallgrass.
D. A large number of small wetlands are found throughout the region.
2. What has taken place in the Great Plains grasslands during the last century?
A. The average annual rainfall has risen and fallen several times.
B. Large parts of the prairie have been converted to agricultural use.
C. Melting glaciers have formed numerous depressions in the soil.
D. Scientists have transformed the region into one large carbon sink.
3. The word alien in paragraph 2 is closest in meaning to
4. The prairie potholes owe their origins mainly to
A. the variation in temperature throughout the year
B. the glaciers that melted at the end of the last Ice Age
C. the heavy rains that fall in the spring and summer
D. the increase in the greenhouse gases in the atmosphere
Direction: Choose one of the topics to write an essay with no less than 250 words within 30 minutes.
1. Do you agree or disagree with the following statement? Parents are the best teachers. Use specific reasons and examples to support your answer.
2. If you could change one important thing about your hometown, what would you change?
3. Do you agree or disagree that it is better for children grow up in the countryside than in a big city?