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Please Fasten Your Seatbelts
Severe turbulence（湍流）can kill aircraft passengers.Now,in test flights over the Rocky Mountains, NASA（美国航空航天局）engineers have successfully detected clear-air turbulence up to 10 seconds before an aircraft hits it.
Clear-air turbulence often catches pilots by surprise.Invisible to radar,it is difficult to forecast and can hurl（用力抛出去）passengers about the cabin.In December 1997 , one passenger died and a hundred others were injured when unexpected rough air caused a United Airlines flight over the Pacific to drop 300 meters in a few seconds.
However,passengers can avoid serious injury by fastening their seatbelts."It is the only antidote（对 策）for this sort of thing,"says Rod Bogue, project manager at NASA's Dryden Flight Research Center in Edwards,California.
The Center's new turbulence detector is based on lidar,or laser-radar.Laser pulses are sent ahead of the plane and these are then reflected back by particles in the air.The technique depends on the Doppler Effect（多普勒效应）.The wavelength of the light shifts according to the speed at which the particles are approaching.In calm air,the speed equals the plane's airspeed.But as the particles swirl（打漩） in rough air,their speed of approach increases or decreases rapidly.The rate of change in speed corresponds to the severity（激烈程度）of the turbulence.
In a series of tests that began last month,a research jet flew repeatedly into、 disturbed air over the mountain ridges（山脉）near Pueblo , Colorado.The lidar detector spotted turbulence between 3 and 8 kilometers ahead, and its forecasts of strength and duration corresponded closely with the turbulence that the plane encountered.
Bogue says that he had"a comfortable amount of time"to fasten his seatbelt.The researchers are planning to improve the lidar's range with a more powerful beam.The system could be installed on commercial aircraft in the next few years.
The last paragraph tells us,among other things,that____＿.
A.the lidar detector needs improvement
B.many airlines are interested in the system
C.passengers often forget to fasten their seatbelts
D.the lidar detector can be used in a wide range of areas
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Exercising Your Memory
1. Aging does not mean a dramatic decline in memory power,unless you help it happen by let-ting your mind go.
2. That's not to say that memory doesn't change throughout life.Researchers divide memory into categories based on the length of time when memories are stored.One system divides it up as short-term(less than one minute;remembering a telephone number while you dial,for in-stance),long-term(over a period of years)and very long-term memory(over a lifetime).
3. Short-term memory isn't mastered until about age 7,but after that you never lose it.Long-term memory,however,involves more effort and skill and changes more through life .It's not until the early teens（十几岁）that most people develop a mature long-term memory.
4. First,we must get information into our heads through learning. Learning strategies can get rusty（生锈）without constant use. High school and college students , who are forced to repeatedly exercise their long-term memory abilities(at least long-term enough to get them through a final exam),usually do well on memory tests.The longer you stay in school,the more chance you get to polish your learning skills.It's no wonder that more highly educated people have more effective memory skills throughout life.
5. Although older people in general learn somewhat more slowly than they did when younger,a dramatic difference exists between those who stay intellectually active-reading,discussing,taking classes,thinking-and those who do not. Giving the brain daily workout（锻炼）is just as important as exercising your muscles.Brainwork keeps your learning strategies in shape,and this helps your memory to function at full capacity.
6. The next part of a healthy long-term memory is retention（记忆力）,the ability to store what you have learned. Memory researchers still do not know whether memories are lost-whether they still exist in the brain but our mental searching cannot turn them up,or have disappeared entirely as our brain ages.
7. The third necessity for memory is recall,the ability to bring to mind the memories we have stored.Again,while aging has widely different effects on the recall abilities of different people, research indicates that the older we get,the longer it takes to recall facts But slower recall is still recall. In fact,aging does not seem to have any effect on forgetting at all,which takes place at the same rate in younger and older people.
Paragraph 6______
A.Importance of staying intellectually active
B.Effects of aging on a person's recall ability
C.Short-term memory versus long-term memory
D.Retention as the second necessity for memory
E.Link between learning strategies and effective memory skills
F.Significance of exercising your muscles
