[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. the emission of carbon dioxide from cars, factories, and power plants
B. the action of carbon dioxide as a greenhouse gas
C. the capture and storage of carbon dioxide in trees, soil, and plants
D. the average amount of carbon dioxide in one acre of land

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