无油润滑压缩机采用聚四氟乙烯塑料代替金属制造密封件，聚四氟乙烯的优点有（）、耐化学腐

下列关于光现象的说法中正确的是

A．光发生漫反射时，反射角有时不等于入射角

B．我们能看到鲜艳的黄色的花是因为花能发出黄色的光

C．彩色电视机的色彩是用红、黄、蓝三种色光按不同的比例混合得到的

D．“背日喷乎水，成霓虹之状”，这是一种光的色散现象

 如图所示的空间分为I、II、III三个区域，各竖直边界面相互平行，I、II区域均存在电场强度为E的匀强电场，方向垂直界面向右；同时II区域存在垂直纸面向外的匀强磁场；III区域空间有一与FD边界成45⁰角的匀强磁场，磁感应强度大小为B，其下边界为水平线DH,右边界是GH:一质量为、电荷量为q的带正电的粒子（重力不计）从O点由静止释放，到达A点时速度为v₀，粒子在C点沿着区域III的磁感线方向进人III区域，在DH上的M点反弹，反弹前、后速度大小不变，方向与过碰撞点的竖直线对称，已知粒子在III区域内垂直于磁场方向的平面内做匀速圆周运动的轨道半径为r= ，C点与M点的距离为，M点到右边界GH的垂直距离为。求：

(1）粒子由O点运动到A点的时间t₁=?

（2) A与C间的电势差U_AC=?

（3）粒子在III区域磁场内运动的时间t₂＝？

阅读理解。

    This year some twenty-three hundred teenagers (young people aged from13~19) from all over the world

will spend about ten months in U.S. homes. They will attend U.S. schools, meet U.S. teenagers, and form

impressions of the real America. At the same time, about thirteen hundred American teenagers will go to other

countries to learn new languages and gain a new understanding of the rest of the world.

     Here is a two-way student exchange in action. Fred, nineteen, spent last year in Germany with George's

family. In turn, George's son Mike spent a year in Fred's home in America. 

     Fred, a lively young man, knew little German when he arrived, but after two months' study the language

began to come to him. School was completely different from what he had expected-much harder. Students

rose respectfully when the teacher entered the room. They took fourteen subjects instead of the six that are

usual in the United States. There were almost no outside activities.

     Family life, too, was different. The father's word was law, and all activities were around the family rather

than the individual. Fred found the food too simple at first. Also, he missed having a car. 

     "Back home, you pick up some friends in a car and go out and have a good time. In Germany, you walk,

but you soon learn to like it."

     At the same time, in America, Mike, a friendly German boy, was also forming his idea. "I suppose I should

criticize (批评) American schools," he said. "It is far too easy by our level. But I have to say that I like it very

much. In Germany we do nothing but study. Here we take part in many outside activities. I think that maybe

your schools are better in training for citizens. There ought to be some middle ground between the two."

1. This year _____ teenagers will take part in the exchange programme between America and other countries. [ ]

A. twenty-three hundred

B. thirteen hundred

C. over three thousand

D. less than two thousand

2. The whole exchange programme is mainly to _____. [ ]

A. help teenagers in other countries know the real America

B. send students in America to travel in Germany

C. let students learn something about other countries

D. have teenagers learn new languages

3. Fred and Mike agree that _____. [ ]

A. America food tasted better than German food

B. German schools were harder than American schools

C. Americans and Germans were both friendly

D. There were more cars on the streets in America

4. What is particular in American schools is that _____. [ ]

A. there is some middle ground between the two teaching buildings

B. there are a lot of after-school activities

C. students usually take fourteen subjects in all

D. students go outside to enjoy themselves in a car

下列哪一项不是NOS提供的基本服务的内容

A．数据库服务

B．网络安全管理服务

C．信息服务

D．分布式服务

武当山中最大的道宫是什么宫（）？

A.太和宫

B.玉虚宫

C.清微宫

D.朝天宫