[单项选择]

Set 1
NORTH AMERICAN GRASSLANDS
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 Mountains 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 40 cm (16 inches) in the west to 80 cm (31 inches) in the east. Average annual temperatures range between 10 degrees and 20 degrees Celsius (50 to 68 degrees Fahrenheit). In the moist regions of the North American 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.
2.One hundred years ago, the Great Plains grasslands were one vast, unbroken prairie. Much of the prairie is now farmland, the m
A. It regulates carbon emissions from human activities.
B. It measures the amount of carbon dioxide in the ecosystem.
C. It collects and drains seasonal rainwater from the soil.
D. It removes carbon dioxide naturally from the atmosphere.

[单项选择]

Set 1
NORTH AMERICAN GRASSLANDS
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 Mountains 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 40 cm (16 inches) in the west to 80 cm (31 inches) in the east. Average annual temperatures range between 10 degrees and 20 degrees Celsius (50 to 68 degrees Fahrenheit). In the moist regions of the North American 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.
2.One hundred years ago, the Great Plains grasslands were one vast, unbroken prairie. Much of the prairie is now farmland, the m
A. Summer temperatures are very hot, while winter temperatures 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.

[填空题]Anthropologists have divided prehistoric North America Indian societies into three categories: bands, tribes, and chiefdoms. Bands were the earliest and most common form of social organization. However, it is unlikely that the first people who came to North America about forty thousand years ago traveled in bands. The climate in the Bering Strait region was so severe and food was so scarce that large groups of people, such as the bands, would have had great difficulty in making such a journey successfully. The earliest people probably traveled in single family units. Some of the units, however, may have gathered together periodically for social reasons.
66. ______
Whatever the beginning of the bands in the New World, this form of organization was very common after the initial settlement of the continent. Bands flourished throughout North America until the time of European colonization. In many areas, they survived long after the arrival of the Europeans. When English, Dut

[填空题]On December 25, 2000, many people across North America received a rare Christmas treat when the moon passed in front of the sun resulting in a partial solar eclipse.
Solar eclipses occur when the moon (47) between the earth and the sun, and the moon’s shadow covers part of the earth; and a total solar eclipse takes place when the moon’s shadow blocks out the sun (48) . What made this particular solar eclipse unique was that this event has occurred on December twenty-fifth only 30 times (49) the past 5000 years, the last time in 1954.
But people must be exceptionally (50) when attempting to view a solar eclipse. Without taking precautionary (51) , one can permanently damage the retina of the eye; however, there are several safe methods of (52) this heavenly marvel.
First, you can view a solar eclipse by using eclipse safety glasses for filtering out the sun’s (53) rays. They should be used when a