21、 阅读材料，回答21-40题。
Trees are useful to man in three important ways: they provide him with wood and other products; they give him shade; and they help to prevent droughts and floods.
Unfortunately, in many parts of the world, man has not realized that the third of these services is the most important. In his eagerness to make money from trees, he has cut them down in large numbers, only to find that without them he has lost the best friends he had. And besides, he is usually too careless to plant and look after new trees. So the forests slowly disappear.
This does not only mean that man will have fewer trees. The results are even more serious for where there are trees, their roots break up soil allowing the rain to sink in and also bind the soil, thus preventing it from being washed away easily; but where there are no trees, the rain fall son hard ground and flows away, causing floods and carrying away the rich topsoil. When all the topsoil is gone, nothing remains but worthless desert.
Two thousand years ago a rich and powerful country cut down its trees to build warships, with which to gain itself an empire. It set up the empire but, without its trees, its soil became poor and it grew weak. When the empire fell to pieces, the home country found itself faced with floods and starvation.
What is the most important service of trees to man according to the passage?
A．They help him make money.
B．They give him wood and other products.
C．They help him prevent droughts and floods.
D．They give him shade.


22、 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.


23、 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 its trees.
D．Because too much had been spent on wars.


24、 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 topsoil.
C．Because too much rain sinks in and washes away the topsoil.
D．Because roots of the trees break up the soil.


25、 What does the author tell us in this passage?
A．How trees help prevent droughts and floods.
B．The relationship between trees and man.
C．How an empire fell to pieces in ancient times.
D．A story of trees.


26、 阅读材料，回答26-45题。
In what now seems like the prehistoric times of computer history, the early postwar era( 战后时期), there was a quite widespread concern that computers would take over the world from man one day. Already today, less than forty years later, as computers are relieving us of more and more of the routine tasks in business and in our personal lives, we are facing with a less dramatic but also less foreseen problem. People tend to be over trusting (过分信任) of computers and are reluctant to challenge their authority. Indeed, they behave as if they were hardly aware that wrong buttons maybe pushed, or that a computer may simply malfunction(失灵).
Obviously, there would be no point in investing(投入) in a computer if you had to check all its answers, but people should also rely on their own internal computers and check the machine when they have the feeling that something has gone wrong. Questioning and routine double checks must continue to be as much a part of good business as they were in pre computer days. Maybe each computer should come with the following warning: for all the help this computer may provide, it should not be seen as a substitute for fundamental thinking and reasoning skills.
What is the main purpose of this passage?
A．To look back to the early days of computers.
B．To explain what technical problems may occur with computers.
C．To discourage unnecessary investment in computers.
D．To warn against the blindness to the probable shortcomings of computers.


27、 The passage recommends those dealing with computers to__________
A．be reasonably doubtful about them
B．check all their answers
C．substitute them for basic thinking
D．use them for business purpose only


28、 An "internal computer" ( Para.2 ) is__________
A．a computer used exclusively by one company for its own problems
B．a person’s store of knowledge and the ability to process it
C．the most up to date in home computer a company can buy
D．a computer from the postwar era which is very reliable


29、 The passage suggests that the present day problem with regard to computers is__________
A．challenging
B．psychological
C．dramatic
D．overtrusting


30、 It can be inferred from the passage that the author would disapprove of__________
A．computer science courses in high schools
B．businessmen and women who use pocket calculators
C．maintenance(连续不断) checks on computers
D．companies which depend entirely on computers