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单选题
Why does land become desert after all trees are cut down?
A
Because nothing remains on land except floods.
B
Because there are no longer trees to keep the rain and protect the top-soil.
C
Because too much rain sinks in and washes away the top-soil.
D
Because roots of the trees break up the soil.
参考答案
参考解析
解析：
推理题。第三段最后一句指出“When all the top-soil is gone, nothing remains but worthless desert.”表层土壤流失导致了沙漠化，而树木是可以防止水土流失的。
推理题。第三段最后一句指出“When all the top-soil is gone, nothing remains but worthless desert.”表层土壤流失导致了沙漠化，而树木是可以防止水土流失的。
更多 “单选题Why does land become desert after all trees are cut down?A Because nothing remains on land except floods.B Because there are no longer trees to keep the rain and protect the top-soil.C Because too much rain sinks in and washes away the top-soil.D Because roots of the trees break up the soil.” 相关考题
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考题
AWhy do plants grow in some places and not in others? Why does some land have so much growing on it，while other land has almost no plants growing on it at all?To grow plants need several things． One is warmth． In very cold places almost nothing grows． Plants also need water． In very dry parts of the earth only a few unusual plants can grow． That's why dry deserts everywhere are almost not covered by trees or grass．Plants must also have a place in which to f，ut down their roots and grow． They find it diffi-culr to grow on hard land． The town is built on hard land． The plants here have only the soil found between the cracks of the stones Io grow in．Another thing plants must have before they can grow is food．Whar will happen if we try to make things grow on the sandy beach? A few plants，such as beach grass，will grow in sand，L ut most plants won-t． Even if the weather is warm enough．and we water the plants each day． Many of them will die because the sand on this beach has al-most no food for plants．( )21． According to the passage，plants need _________ things to grow well．A． twoB． threeC． fourD． five
考题
BScientists are trying to make the deserts into good land again． They want to，bring water to the deserts． But more and more places of the earth is becoming deserts all the time， Scientists may not be able to change the deserts． Scientists think that people make deserts． People are do-ing bad things to the earth．Some places on the earth don-t get very much rain． But they still don-t become deserts．This is because some green plants are growing there． Small green plants and grass are very helpful to dry places． Plants don't let the wind blow the earth away． When 8 little bit of rain falls，the plants hold the water． Without plants，the land can become a desert much more easily．( )26． Why can the land become a desert?A． Because some places don't get very much rain．B． Because plants don-t let the hot sun make the earth even drier．C． Because people don't keep the earth well．D． Because scientists haven-t learnt enough about the deserts．
考题
共用题干
Why Buy Shade-Grown Coffee?When people argue about whether coffee is good for health,they're usually thinking of the health of the coffee drinker. Is it good for your heart?Does it increase blood pressure?Does it help you concentrate?However,coffee affects the health of the human population in other ways, too.Traditionally,coffee bushes were planted under the canopy（树冠）of taller indigenous（土生土长的）trees. However, more and more farmers in Latin America are deforesting the land to grow full-sun coffees.At first,this increases production because more coffee bushes can be plan- ted if there aren't any trees.With increased production come increased profits.Unfortunately,deforesting for coffee production immediately decreases local wildlife habitat Native birds nest and hide from predators（捕食者）in the tall trees and migrating birds rest there.Furthermore,in the long term,the full-sun method also damages the ecosystem because more chemical fertilizers and pesticides are needed to grow the coffee.The fertilizers and pesti- cides kill insects that eat coffee plant,but then the birds eat the poisoned insects and also die. The chemicals kill or sicken other animals as well,and can even enter the water that people will eventually drink.Fortunately,farmers in Central and South America are beginning to grow more coffee bushes in the shade.We can support these farmers by buying coffee with such labels as“shade grown” and“bird friendly.”Sure,these varieties might cost a little more.But we're paying for the health of the birds,the land,ourselves,and the planet. I think it's worth it. How do farmers find more land for growing full-sun coffee?A: They buy more land from other farmers.B: They cut down trees.C: They move to another country.D:.They turn grassland into farmland.
考题
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
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After many years' wandering all over the world,the man finally settled down to the business his father left to him.A:focused on
B:depended on
C:insisted on
D:dwelled on
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考题
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Horizontal movement of the water toward the land after high tideB
Horizontal movement of the water toward the land after low tideC
Horizontal movement of the water away from the land following high tideD
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考题
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In the absence of a terminating action, all routes are accepted.B
The route will be evaluated by the second term in the policy.C
The default action will be applied.D
In the absence of a termination action, all routes are rejected.
考题
单选题What doesn’t the old tree tell us?A
He’s afraid to be cut down somedayB
He has fewer and fewer friendsC
The trees have the longest lives in the worldD
He has lived for thousands of years
考题
单选题______ of the land in that district is covered with trees and grass.A
Fifth twoB
Two fifthC
Fifth secondD
Two fifths
考题
单选题Why do forests in many parts of the world slowly disappear?A
New trees are not looked after properly.B
Many trees have been cut down by man.C
Man has not paid enough attention to planting trees.D
All of the above.
考题
单选题Which statement about PIM sparse mode operations is true？（）A
It does not support all underlying unicast routing protocols like BGPB
Receivers are registered with RP by their first hop routerC
From the RP， traffic flows down a source tree to each receiver.D
It supports shared trees only， and assumes all hosts want the multicast trafficE
Receivers are joined to the shared tree by their local designated router
考题
单选题Why did the country mentioned in the passage suffer from floods and starvation?A
Because an empire was set up.B
Because the empire fell to pieces.C
Because it lost all its trees.D
Because too much had been spent on wars.
考题
单选题Countries sometimes spoil their beauty spots because ______.A
they are too poor to build beautiful hotels on themB
they have to cut down trees to build hotels on themC
they have to sell them to borrow money from foreign banksD
they are lacking experience in building hotels
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