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What geologists call the Basin and Range Province in the United States roughly
coincides in its northern portions with the geographic province known as the Great
Basin. The Great Basin is hemmed in on the west by the Sierra Nevada and on the east
by the Rocky Mountains; it has no outlet to the sea. The prevailing winds in the Great
(5) Basin are from the west. Warm, moist air from the Pacific Ocean is forced upward as it
crosses the Sierra Nevada. At the higher altitudes it cools and the moisture it carriers is
precipitated as rain or snow on the western slopes of the mountains. That which
reaches the Basin is air wrung dry of moisture. What little water falls there as rain or
snow, mostly in the winter months, evaporates on the broad, flat desert floors. It is,
(10) therefore, an environment in which organisms battle for survival. Along the rare
watercourses, cottonwoods and willows eke out a sparse existence. In the upland
ranges, pinon pines and junipers struggle to hold their own.
But the Great Basin has not always been so arid. Many of its dry, closed depressions
were once filled with water. Owens Valley, Panamint Valley, and Death Valley were
(15) once a string of interconnected lakes. The two largest of the ancient lakes of the Great
Basin were Lake Lahontan and Lake Bonneville. The Great Salt Lake is all that
remains of the latter, and Pyramid Lake is one of the last briny remnants of the former.
There seem to have been several periods within the last tens of thousands of
years when water accumulated in these basins. The rise and fall of the lakes were
(20) undoubtedly linked to the advances and retreats of the great ice sheets that covered
much of the northern part of the North American continent during those times. Climatic
changes during the Ice ages sometimes brought cooler, wetter weather to midlatitude
deserts worldwide, including those of the Great Basin. The broken valleys of the Great
Basin provided ready receptacles for this moisture.
41.What is the geographical relationship between the Basin and Range Province and the Great Basin?
(A) The Great Basin is west of the Basin and Range Province.
(B) The Great Basin is larger than the Basin and Range Province.
(C) The Great Basin is in the northern part of the Basin and Range Province.
(D) The Great Basin is mountainous; the Basin and Range Province is flat desert.
47.Why does the author mention Owens Valley, Panamint Valley, and Death Valley in the second paragraph?
(A) To explain their geographical formation
(B) To give examples of depressions that once contained water
(C) To compare the characteristics of the valleys with the characteristics of the lakes
(D) To explain what the Great Basin is like today
48.The words "the former" in line 17 refer to
(A) Lake Bonneville
(B) Lake Lahontan
(C) The Great Salt Lake
(D) Pyramid Lake
以上三个选什么呢?_?
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