填空题The quick-growing exotic tree species are replanted in the deforested land because they can help prevent soil erosion.____

共用题干
Ants as a Barometer of Ecological Change
At picnics,ants are pests.But they have their uses.In industries such as mining,farming and forestry,they can help gauge the health of the environment by just crawling around and being antsy.
It has been recognized for decades that ant—which are highly sensitive to ecological change—can provide a near-perfect barometer of the state of an ecosystem.Only certain species, for instance,will continue to thrive at a forest site that has been cleared of trees._______(46) And still others will move in and take up residence.
By looking at which species populate a deforested area,scientists can determine how “stressed”the land is._______(47)Ants are used simply because they are so common and comprise so many species.
Where mine sites are being restored,for example,some ant species will recolonize the stripped land more quickly than others._______(48)Australian mining company Capricorn Coal Management has been successfully using ant surveys for years to determine the rate of recov-ery of land that it is replanting near its German Creek mine in Queensland.
Ant surveys also have been used with mine-site recovery projects in Africa and Brazil,where warm climates encourage dense and diverse ant populations.“We found it worked extremely well there,”says Jonathan Majer,a professor of environmental biology.Yet the surveys are perfectly suited to climates throughout Asia,he says,because ants are so common throughout the region. As Majer puts it:“That's the great thing about ants.”
Ant surveys are so highly-regarded as ecological indicators that governments worldwide accept their results when assessing the environmental impact of mining and tree harvesting._______ (49)
Why not?Because many companies can't afford the expense or the laboratory time needed to sift results for a comprehensive survey.The cost stems,also,from the scarcity of ant specialists. ______(50)

_______(46)
A: This allowed scientists to gauge the pace and progress of the ecological recovery.
B: Yet in other businesses,such as farming and property development,ant surveys aren't used widely.
C: Employing those people are expensive.
D: They do this by sorting the ants,counting their numbers and comparing the results with those of earlier surveys.
E: The evolution of ant species may have a strong impact on our ecosystem.
F: Others will die out for lack of food.

Although I sympathize,I can't really do very much to help.

A:Because
B:Since
C:Though
D:For

Deforestation and Desertification(沙漠化)
　　The Sahel zone lies between the Sahara desert and the fertile savannahs(热带大草原)of northern Nigeria and South Sudan. The word sahel comes from Arabic and means marginal or transitional ,and this is a good description of the semi-arid(半干旱) lands, which occupy much of the Western African countries of Mail, Mauritania, Niger ,and Chad.
　　Unfortunately, over the last century the Sahara desert has steadily crept southwards eating into once productive Sahel lands. United Nations surveys show that over 70 percent of the dry land in agriculture use in Africa has deteriorate dover the last 30 years. Droughts have become more severe, the most recent lasting over twenty years in parts of the Sahel region. The same process of desertification is taking place across southern Africa as the Kalahari desert advances into Botswana and parts of South Africa.
　　One of the major causes of this desert advance is poor agricultural land use, driven by the pressures of increasing population. Overgrazing一 keeping too many farm animals on the land一means that grasses and other plants cannot recover, and scarce water supplies are exhausted. Over cultivation一 trying to grow too many crops on poor land一 results in the soil becoming even less fertile and drier, and beginning to break up. Soil erosion (侵蚀) follows, and the land turns into desert.
　　Another cause of desertification is loss of tree cover. Trees are cut down for use as fuel and to clear land for agricultural use. Tree roots help to bind the soil together, to conserve moisture, and to provide a habitat for her plants and animals. When trees are cut down, the soil begins to dry and loosen, wind and rain erosion increase, other plant species die, and eventually the fertile top soil may be almost entirely lost, leaving only bare rock and dust.
　　The effects of loss of topsoil and increased drought are irreversible. They are, however, preventable. Careful conservation of tree cover and sustainable agricultural land use have been shown to halt deterioration of soils and lessen the effects of shortage of rainfall. One project in Kita in south-west Malifunded by UNDP has involved local communities in sustainable management offorest, while at the same time providing a viable(有活力的)agricultural economy. This may be a model for similar projects in other West African countries.
文章{1~5)

The Sahel zone is an area which _____.

A.is covered with sad and grass
B.has a long history
C.occupies much of South Nigeria
D.belongs to Sudan

共用题干
Ants as a Barometer of Ecological Change
At picnics,ants are pests.But they have their uses.In industries such as mining,farming and forestry , they can help gauge（测定）the health of the environment by just crawling around and being antsy（坐立不安）.
It has been recognized for decades that ants—which are highly sensitive to ecological change—can provide a near-perfect barometer（气压计）of the state of an ecosystem. Only certain species,for instance,will continue to thrive at a forest site that has been cleared of trees. ______(46)And still others will move in and take up residence.
By looking at which species populate a deforested area,scientists can determine how
“stressed”the land is.______(47)Ants are used simply because they are so common and comprise（构成）so many species.
Where mine sites are being restored,for example,some ant species will recolonize（重新移居到.……）the stripped land more quickly than others.______ (48 ) Australian mining compa- ny Capricorn Coal Management has been successfully using ant surveys for years to determine the rate of recovery of land that it is replanting near its German Creek mine in Queensland.
Ant surveys also have been used with mine-site recovery projects in Africa and Brazil,where warm climates encourage dense and diverse ant populations.“We found it worked extremely well there,”says Jonathan Major,a professor of environmental biology.Yet the surveys are perfectly suited to climates throughout Asia,he says,because ants are so common throughout the region.As Major puts it:“That's the great thing about ants.”
Ant surveys are so highly regarded as ecological indicators（指示器）that governments world-wide accept their results when assessing（评估）the environmental impact of mining and tree har- vesting.______(49）.
Why not?Because many companies can't afford the expense or the laboratory time needed to sift（详审）results for a comprehensive survey. The cost stems,also, from the scarcity（缺乏）of ant specialists.______(50)

______(47)
A:This allowed scientists to gauge the pace and progress of the ecological recovery.
B:Yet in other businesses,such as farming and property development,ant surveys aren't used widely.
C: Employing those people are expensive.
D: They do this by sorting the ants,counting their numbers and comparing the results with those of earlier surveys.
E: The evolution of ant species may have a strong impact on our ecosystem.
F: Others will die out for lack of food.