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[主观题]

Under proper conditions, sound waves will be reflected from a hillside or other such obstr

uction. Sound travels at the rate of about one-fifth of a mile per second. If the hill is eleven hundred feet away, it takes two seconds for the sound to travel to the hill and back. Thus, by timing the interval between a sound and its reflection (the echo) , you can estimate the distance to an obstruction.

During World War II the British used a practical application of this principle to detect German planes on their way to bomb London long before the enemy was near the target. They used radio waves instead of sound waves, since radio waves can penetrate fog and clouds. The outnumbered Royal Air Force(RAF) always seemed to the puzzled Germans to by lying in wait at the right time and never to be surprised. It was radio echoes more than anything else that won the Battle of Britain.

Since the radio waves were used to tell the direction in which to send the RAF planes and the distance to send them (their rage of flight, in other words) , the device was called "radio directing and ranging" , and from the initials the word "radar" was coined.

Sound waves reflected from a hill can be used to estimate the______.

A.height of the hill

B.speed of sound

C.distance to the hill

D.intensity of sound

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更多“Under proper conditions, sound waves will be reflected from a hillside or other such obstr”相关的问题

第1题

If any contracting party considers that a product is not receiving from another contracting party th
e treatment which the first contracting party believes to have been contemplated by a concession provided for in the appropriate Schedule annexed to this agreement,it shall bring the matter directly to the attention of other contracting party.If the latter agrees that the treatment contemplated was that claimed by the first contracting party,but declares that such treatment cannot be accorded because a coun or other proper authority has ruled to the effect that the product involved cannot be classified under the tariff laws of such contracting party so as to permit the treatment contemplated in this Agreement,the two contracting parties,together with any other contracting parties substantially interested,shall enter promptly into further negotiation with a view to compensatory adjustment of the matter.
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第2题

According to the passage, why did women become active in politics?A.To improve the conditi

According to the passage, why did women become active in politics?

A.To improve the conditions of life that existed at the time.

B.To support Elizabeth Cady Stanton for president.

C.To be elected to public office.

D.To amend the Declaration of Independence.

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第3题

We don't think we can put the business through( )you revise your terms and conditi
We don't think we can put the business through()you revise your terms and conditi

We don't think we can put the business through()you revise your terms and conditions.

A.unless

B.expect

C.only if

D.in addition

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第4题

Part ADirections: Read the following four texts. Answer the questions below each text by c

Part A

Directions: Read the following four texts. Answer the questions below each text by choosing A, B, C or D. (40 points)

Education is not an end, but a means to an end. In other words, we do not educate children only for the aim of educating them. Our purpose is to fit them for life. Life is varied; so is education. As soon as we realize the fact, we will understand that it is very important to choose a proper system of education.

In some countries with advanced industries, they have free education for all. Under this system, people, no matter whether they are rich or poor, clever or foolish, have a chance to be educated at universities or colleges. They have for some time thought, by free education for all, they can solve all the problems of a society and build a perfect nation. But we can already see that free education for all is not enough. We find in such countries a far larger number of people with university degrees than there are jobs for them to fill. As a result of their degrees, they refuse to do what they think is "low" work. In fact, to work with one's hands is thought to be dirty and shameful in such countries.

But we have only to think a moment to understand that the work of a completely uneducated farmer is as important as that of a professor. We can live without education, but we should die if none of us grow crops. If no one cleaned our streets and took the rubbish away from our houses, we should get terrible diseases in our towns. If there were no service people, because everyone was ashamed to do such work, the professors would have to waste much of their time doing housework.

On the other hand, if all the farmers were completely uneducated, their production would remain low. As the population grows larger and larger in the modern world, we would die if we did not have enough food.

In fact, when we say all of us must be educated to fit ourselves for life, it means that all must be educated: firstly, to realize that everyone can do whatever job is suited to his brain and ability; secondly, to understand that all jobs are necessary to society and that it is had to be ashamed of one's own work or to look down upon someone else's; thirdly, to master all the necessary know-how to do one's job well. Only such education can be called valuable to society.

Our purpose of educating children is to______.

A.accustom them to varied life

B.choose a proper system of education

C.educate them only for the aim of educating them

D.make them intelligent citizens

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第5题

Under pressure from animal welfare groups, two national science teachers' associations hav
e adopted guidelines that ban classroom experiments harming animals. The National Association of Biology Teachers and the National Science Teachers' Association hope to end animal abuse in elementary and secondary schools and, in turn, discourage students from mishandling animals in home experiments and science fair projects.

Animal welfare groups are apparently most concerned with high school students experimenting with animals in extracurricular projects. Barbara Orlans, President of the Scientists' Center for Animal Welfare, said that students have been performing surgery at random, testing known poisonous substances, and running other pathology (病理学) experiments on animals without even knowing normal physiology (生理学).

At one science fair, a student cut off the leg and tail of a lizard (蜥蜴) to demonstrate that only the tail can regenerate, she said. In another case, a student bound sparrows, starved them and observed their behavior.

"The amount of abuse had been quite horrifying," Orlans said.

Administrators of major science fairs are short-tempered over the teachers' policy change and the impression it has created. "The teachers were sold a bill of goods by Barbara Orlans," said Thurman Grafton, who heads the rules committee for the International Science and Engineering Fair. "Backyard tabletop surgery is just nonsense. The new policies throw cold water on students inquisitiveness," he said. Grafton said he wouldn't deny that there hasn't been animal abuse among projects at the international fair, but he added that judges reject contestants who have unnecessarily injured animals. The judges have a hard time monitoring local and regional fairs that may or may not choose to comply with the international fair's rules that stress proper care of animals, Grafton said.

He said that several years ago, the Westinghouse Science Talent Search banned harmful experiments to animals when sponsors threatened to cancel their support after animal welfare groups lobbied for change.

The teachers adopted the new policies also to fend off proposed legislation — in states including Missouri and New York — that would restrict or prohibit experiments on animals.

Officials of the two teachers' organizations say that they don't know how many animals have been abused in the classroom. On the one hand, many biology teachers are not trained in the proper care of animals, said Wayne Moyer, executive director of the biology teachers' association. On the other hand, the use of animals in experiments has dropped in recent years because of school budget cuts. The association may set up seminars to teach better animal care to its members.

What is the passage mainly about?

A.Science teachers banning testing harmful to animals.

B.Teachers' policy change in experiment on animals.

C.The new policies of banning harmful experiments to animals.

D.The importance of prohibiting harmful experiments on animals.

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第6题

Prolonged and excessive use of alcohol can seriously undermine an individual's health. Phy
sical deterioration occurs. Large quantities of alcohol can directly damage body tissue and indirectly cause malnutrition. Nutritional deficiencies can result for several reasons. Alcohol contains empty calories, which have no significant nutritive value. When consumed in substantial amounts, alcohol curbs one's appetite for more wholesome foods. Excessive alcohol intake can interfere with the proper digestion and absorption of food. Therefore, even the heavy drinker who does eat a well-balanced diet is deprived of me essential nutrients. Maintenance of a drinking habit can deplete economic resources otherwise available for buying good, wholesome food. Malnutrition itself further reduces the body's ability to utilize the nutrients consumed. The result of damaged tissue and malnutrition can be brain injury, heart disease, diabetes, or cancer of the liver, and weakened muscle tissue. Untreated alcoholism can reduce one's life span by ten to twelve years.

Heavy alcohol consumption also affects the body's usage of other drugs and medications. The dosages required by excessive drinkers may differ from those required by normal or non-drinkers. Serious consequences can be incurred unless the prescribing physician is aware of the patient's drinking habits.

Sudden death may result from excessive drinking. It might occur when the individual has ingested such a large amount of alcohol that the brain center controlling breathing and heart action is adversely affected, or when taking some other drugs, particularly sleep preparations along with alcohol. Death, as a result of excessive drinking, can come during an automobile accident since half of all fatal traffic accidents involve the use of alcohol. Many self-inflicted deaths, as well as homicides, involve the use of alcohol.

It is important to remember that alcohol is a drug that is potentially addictive. Once the user is hooked on alcohol, withdrawal symptoms occur when it is not sufficiently available to body cells. At the onset of developing alcohol addiction, these symptoms may be relatively mild and include hand tremors, anxiety, nausea, and sweating. As dependency increases, so does the severity of the withdrawal syndrome and the need for medical assistance to cope with it.

In 1956 the American Medical Association supported the growing acceptance of alcoholism as an illness, falling under the treatment jurisdiction of the medical profession. Since then, the medical resources for problems of acute and chronic intoxication have increased and improved.

To develop his point, the author has used______.

A.sensory words

B.convincing arguments

C.scientific facts

D.figures of speech

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第7题

A. popular B. proper C. inner D. educated

A.popular

B. proper

C. inner

D. educated

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第8题

They struck a proper balance between work and ________.

A.free

B.leisurely

C.leisure

D.freedom

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第9题

Proper, scientific study of the impacts of dams and of the cost and benefits of contro
lling water can help to resolve these conflicts.

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第10题

A proper title for the passage can be______.A.The Magic of GlueB.The Way to SuccessC.Why T

A proper title for the passage can be______.

A.The Magic of Glue

B.The Way to Success

C.Why Talented People often Succeed Quickly

D.How Average Achievers Do Better than Others

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