第三部分  概括大意与完成句子

（两篇）

第四篇

Compact Disks

1　If someone says to you your music CDs don't really hold any music on them, and they only have numbers recorded on them, you may not believe it. In fact, he is right in that sound is actually recorded onto the CDs as special numbers - a digital code1. The code is pressed onto the CD as bumps on a long spiral track almost five kilometers long. These bumps are an average of 0.5 microns wide

2　A small laser beam shines onto the bumps as the CD turns. The light is reflected back to a receiver that records how the laser light bounces back. This lets the CD player2 turn the reflected light back into the original code. This means you can hear the original code as music.

3　Digital codes are used with many technologies. E-mail needs these kinds of code numbers. Space probes communicate with their ground station on earth using digital codes. Bar codes are read as digital codes in computer systems. Digital communications with cell phones need digital codes. Weather radios also tune in to specific signals using these codes.

4　There are many types of compact disks. One format is called CD-RWs. They can be recorded on and re-recorded on (rewritten on) as you would do with a floppy disk3. Another format is the CD-ROM. The technology for recording on these disks is different from other CDs. These CDs have a dye layer that the CD writer can darken or leave clear. The clear and dark spots are the digital code. CD-ROM stands for Compact Disc - Read Only Memory4. This disk is like a "super" floppy disk that can hold lots of information. One CD-ROM can hold the same amount of data as 500 floppy disks.  Information is permanently recorded onto it. Computer games and other programs are considered to be CD-ROMs.

5　CDs were first sold to the public in 1982. These CDs still play well and sound fine. Current CDs are expected to last between 70 to 200 years. Of course, you can make sure your CDs last a long time by taking care of them.

6　Science keeps on developing. It may not be many more years before a completely new technology is invented5 and introduced to the public for music recording. In the meantime, there is no doubt you will continue to enjoy listening to your favorite music on CDs6 and playing your favorite computer games on CD-ROMs.

词汇：

　　bump n. 隆块，隆起

　　floppy disk 软（磁）盘

　　spiral adj. 螺旋形的

　　format n. 格式

　　probe n. 探测器

　　dye n. 染色

　　floppy adj. 松软的

　　permanently adv. 永久地