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 code.The code is pressed onto the CD as bumps（隆起）on a long spiral track almost five kilometers long.These bumps are an average of 0.5 microns（微米）wide.
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 player turn the reflected light back into the original code.This means you can hear the original code as music.
Digital codes are used with many technologies.E-mail needs these kinds of code numbers.Space probes （探测仪）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.
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 disk（软盘）.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 Memory.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.
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.
Science keeps on developing.It may not be many more years before a completely new technology is invented 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 CDs and play your favorite computer games on CD-ROMs. CDs can last a long time if______.
根据第三段第一句“Digital codes are used with many technologies.”可知，第三段主要讲的是数字代码的用处，紧接着又举电子邮件的例子来对数字代码的用处进行说明。
根据第四段第三句可知，CD-RWs可以被重写。第四段第八句中的“CD-ROM stands for Compact Disk-Read Only Memory”则表明，CD-ROMs指的是“只读存储光盘”，即只能读取而不能重写的光盘。由此可知，两者的区别在于能否被重写。