书放在水平桌面上，桌面会受到弹力的作用，产生这个弹力的直接原因是（ ）
A．书的形变
B．桌面的形变
C．书和桌面的形变
D．书受到的重力
A、书的形变产生的力是书本对桌面的压力，施力物体是书，受力物体是桌面．选项A正确．
B、桌面的形变产生的力是桌面对书的支持力，施力物体是桌面，受力物体是书．选项B错误．
C、书和桌面的形变分别产生了书对桌面的压力和桌面对书的支持力．选项C错误．
D、书受到的重力致使书压紧了桌面，重力的施力物体是地球，受力物体是书．选项D错误．
故选A．
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| If we were asked exactly what we were doing a year ago, we should probably have to say that we could not remember. But if we had 1 a book and had written in it an account (叙述) of what we did each day, we should be able to give an answer to the question. It's the 2 in history. Many things have been 3 because we do not have any written account of them. sometimes men did 4 the most important happenings in their country, but often the material was destroyed by fire or in a war. Sometimes there was never any written record at all 5 the people of that time and place did not know 6 to write. For example, we know a good deal about the people who lived in China 4,000 years ago,because they could write and leave written records for those who lived 7 them. But we know almost nothing about the people who lived even 200 years ago in central Africa, because they had not learned to write. Sometimes, of course, 8 the people cannot write, they may know something of the past. They have heard about it from older people,and often songs and dances and stories have been made about the most important happenings, and these have been sung and acted and told for many 9 . For most people are proud to tell what their fathers did in the past. Such 10 stories are often very helpful where there are no written records. |
| ( )1．A．bought ( )2．A．question ( )3．A．forgotten ( )4．A．experience ( )5．A．although ( )6．A．how ( )7．A．on ( )8．A．while ( )9．A．people ( )10．A．long | B．written B．same B．checked B．know B．when B．what B．by B．since B．generations B．different | C．kept C．story C．damaged C．study C．unless C．when C．after C．even if C．reasons C．personal | D．opened D．mystery D．imagined D．record D．because D．why D．with D．as long as D．purposes D．spoken |