问题 选择题

编号1,2,3,4,5,6的六个球分别放入编号为1,2,3,4,5,6的六个盒子中,其中有且只有三个球的编号与盒子的编号一致的放法种数有(  )

A.20

B.40

C.120

D.480

答案

从6个盒子中选出3个,填入3个球,使三个球的编号与盒子的编号一致,有 C63 种方法,

剩余的3个盒子的编号与三个球的编号不一致,有2种方法,

故有且只有三个球的编号与盒子的编号一致的放法种数有C63×2=40种,

故选 B.

单项选择题
问答题

Internet is a vast network of computers that connects many of the world’s businesses, institutions, and individuals. (46) The Internet, which means interconnected network of networks, links tens of thousands of smaller computer networks. These networks transmit huge amounts of information in the form of words, images, and sounds.

The Internet has information on every topic.(47)Network users can search through sources ranging from vast databases to small electronic" bulletin boards", where users form discussion groups around common interests. Much of the Internet’s traffic consists of messages sent from one computer user to another. These messages are called electronic mail or e-mail. Internet users have electronic addresses that allow them to send and receive e-mail. Other uses of the network include obtaining news, joining electronic debates, and playing electronic games.(48)One feature of the Internet, known as the World Wide Web, provides graphics, audio, and video to enhance the information in its documents. These documents cover a vast number of topics.

People usually access the Internet with a device called a modem. Modems connect computers to the network through telephone lines. Much of the Internet operates through worldwide telephone networks of fiber-optic cables(49) These cables contain hair-thin strands of glass that carry data as pulses of light. They can transmit thousands of times more data than local phone lines, most of which consist of copper wires.

In the future, the internet will probably grow more sophisticated as computer technology becomes more powerful. (50)Many experts believe the Internet may become part of a larger network called the information superhighway. This network, still under development, would link computers with telephone companies, cable television stations, and other communication systems. People could bank, shop, watch TV, and perform many other activities through the network.

(47)Network users can search through sources ranging from vast databases to small electronic" bulletin boards", where users form discussion groups around common interests.