Fill in each of the blanks with one of the words given in the following list,making changes if necessary: commercial dis c floppy optical retrieve erased flat surface access store laser information light reflected represents equivalent CD-ROM stands for compact dis c read-only memory. Unlike 1 and hard disks, which use magnetic charges to represent 1s and 0s, 2 dis c s use reflected light. On a CD-ROM dis c , 1s and 0s are represented by 3 areas and bumpy (高低不平的 ) areas (called “pits”) on its bottom 4 . The CD-ROM dis c is read by a 5 that projects a tiny beam of 6 on these areas. The amount of 7 light determines whether the area 8 a 1 or a 0. Like a 9 CD found in music stores, a CD-ROM is a “read-only” 10 . Read-only means it cannot be written on or 11 by the user. Thus, you as a user have 12 only to the data imprinted ( 压印 ) by the publisher. A single CD-ROM dis c can 13 650 megabyte s ( 兆字节 ) of data. That is 14 to 451 floppy disks. With that much 15 on a single dis c , the time to 16 or access the information is very important. An important characteristic of CD-ROM drives is their access rate.