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90°射频脉冲激发后，组织中将产生宏观横向磁化矢量，射频脉冲关闭后，由于主磁场的不均匀造成了质子群失相位，组织中的宏观横向磁化矢量逐渐衰减。到TE/2时刻，施加一个180°聚相脉冲，质子群逐渐聚相位，组织中宏观横向磁化矢量逐渐增大；到了TE时刻，质子群得以最大限度聚相位，组织中宏观横向磁化矢量达到最大值，从此刻开始，质子群又逐渐失相位，组织中的横向宏观磁化矢量又逐渐衰减。 该序列中90°脉冲的作用是
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In 1971 the Royal College of Physicians decided to take action on the problem of smoking and health. They formed an organization called Action on Smoking and Health -- ASH. Its aims are to make people aware of the dangers of smoking and to help those people who do smoke to give up. As part of its campaign to make people see how smoking damages us all, both as individuals and as members of society, ASH has gathered together some every convincing facts and figures. For in stance, smoking is very harmful to industry. Nearly 50 million working days are lost in Britain every year due to smoking -- at least four times the number of days lost in strikes. And the cost of treating people suffering from smoking related disease is around ￡ 100 million each year.But the human suffering which smoking causes is far more serious than the economic price which society has to pay. 95% of those who suffer from bronchitis are smokers. It has been estimated that 20% of all cigarette smokers become disabled by chronic bronchitis and may eventually die of the disease. Over 20,000 people die of bronchitis every year.Lung cancers kill more people than any other types; of cancers; 90% of them are caused by smoking. With coronary heart disease the risk of death is 2 to 3 times as great in smokers as in non - smokers and this disease is now the major cause of death in Britain.Bronchitis, lung cancer and coronary heart disease arc the three most serious reasons for not smoking but there are other masons which are also important. The message from ASH is clear -- if you do smoke you should give up now and if you don’t smoke then you should not start -- ever. According to the passage,（）are lost in strike.
存储器的技术指标包括 【10】 、存取时间和存取周期、可靠性、功耗。
宽带无线接入技术主要有本地多点分配业务(LMDS)和 【20】 两种。
[听力原文]8-10W: It’s almost vacation time. Have you found a summer job yetM: I suppose I can work at the boy’s camp where I worked last summer. But camp jobs don’t pay much.W: I think I can get a job at the Edgewater Hotel. A friend of mine was a waiter there last summer. The pay wasn’ t good, but he got lots of tips.M: My sister worked there last summer, making beds and cleaning bathrooms. She didn’t like it, but she earned quite a lot of money.W: A friend of my sister’ s did that one summer.M: What I want is a job outside. After sitting in college classes all winter, I’ d like a job in the open air.W: The high school kids earn a lot of money every summer cutting grass. My brother is only fourteen, but he gets five dollars every time he cuts somebody’ s grass, and it only takes him an hour. He just rides around on the machine that he bought, and the machine does all the work.M: That’ s pretty good. I used to cut grass when I was in high school. But now I thought I might work for a road-building company, or something like that.W: It would be a good experience. You could earn a lot, too. What kind of job does the man prefer（）
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