非处方药有哪些特点?

如图，在△ABC中，∠C=90°，M为AB的中点，DM⊥AB，CD平分∠ACB，求证：MD=AM．

某人在室内以窗户为背景摄影时，恰好把窗外从高处落下的一个小石子摄在照片中，已知本次摄影的曝光时间为0.02s，量得照片中石子运动痕迹的长度为1.6cm，实际长度为100cm的窗框在照片中的长度为4cm．凭以上数据，你知道这个石子闯入镜头时已经运动了多长时间吗？（g=10m/s²）（　　）

A．1s

B．2s

C．3s

D．4s

为什么隔爆外壳能起隔爆作用？

At Columbia University, where I taught economics for many years before coming to China, most of my students spent a great deal of time in volunteer work. They taught poor children in the local neighborhoods, they visited the elderly in hospitals and at home and helped them with their shopping, they worked to preserve historic sites and places of beauty, they cleaned up waste dumps, they prepared food for the hungry, they created and ran student newspapers, they organized concerts and artistic events, they acted as translators for migrant workers, they formed political pressure groups, they raised money to combat AIDS, malaria, and other diseases, and so on.
As part 0f that tradition I do volunteer work here in Beijing, just as I did in New York, but I find that my students at Tsinghua University and at other schools in Beijing are much less involved in volunteering then I had expected; In part, of course, this reflects the heavier workload in Chinese schools, which leaves less time for outside activities. But I think there is more to it than just this. I think it also reflects a very different system, in which volunteer work for students here is usually organized or sponsored by government, schools, or other official groups, rather than by the students themselves. (41) .
This is unfortunate. I think it would be better both for them and for society if Chinese students took the initiative to decide what kinds of problems or issues they felt to be of importance, and then played a more active role in organizing the work. (42) .
But there are at least two other important reasons for doing volunteer work. The first is that you can learn a lot about yourself and about your abilities by organizing, taking on responsibilities, deciding on objectives, and fulfilling them. (43) .
This is an important lesson. Many of my students here work very hard, but their attitude towards their work is not always a healthy one. They do the work not because they love it or feel that it is exciting but rather because it is expected of them, and they will get rewarded (or at least not punished) if they do it. With charitable work there is no explicit reward. You work because you have goals, and in the end the only judge of your work is yourself. (44) . It also forces you to think about what you are doing and the best way to accomplish your objectives. You are no longer simply doing something because your teacher or your boss told you to do it.
The second important reason for charitable work is that it changes your relationship with your society. Sometimes I feel that many of the people I meet here don’t really appreciate the greatness of China and the excitement of the process through which it is currently living. The Chinese are well-known for being nationalistic, but sometimes I think this nationalism has more to do with distrust of foreigners than with love of country. Many of my friends and students simply do not know very much about their own country, and often seem unhappy with or embarrassed by certain aspects of China.
(45) . For example, if you help the children of migrant workers with their education, you will quickly realize that poor migrant workers should not be seen as an embarrassment to Beijing. On the contrary, they are a great strength, and their stories are part of a huge and dramatic experiment that China is undergoing. In a small way by working with migrant children you can help make the experiment a success.
[A] I think that if they had spent more time engaged in activities outside of school and family such as doing volunteer work, they would feel very differently.
[B] This changes the way you think about work and about your responsibilities to yourself and others.
[C] You will realize how future events can have just as Big an impact on your life and those of your friends and family.
[D] This means that many students here in Beijing think of volunteering as something that must be done to please teachers, bosses, or other figures of authority, rather than because of a desire to address a problem about which they have thought very deeply.
[E] When you work closely with others who are less fortunate than you, or when you set a local goal and work to accomplish it, you see directly how your actions can affect the world around you.
[F] If you are interested in volunteer work you don’t have to wait for your school, your teachers, or officials on television to tell you what to do.
[G] There are many reasons why this would be good. The most obvious reason, of course, is that we all have obligations towards our society, and volunteer work is one way of repaying this obligation.