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29.(AB) reliable (AC) label (AD) invented (AE) measurement (BC) technically (BD) digitally (BE) protective (CD) remotely (CE) middle (DE) primitive (ABC) player (ABD) converted

(CD) is a thin, circular disc of metal and plastic about 12cm in diameter. It’s actually made of three layers. A CD is made mostly from a plastic base called polycarbonate. Sandwiched in the__21__ there is a thin layer of aluminum. Finally, on top of the aluminum, is a__22__layer of polish to prevent it from being scratched easily. CDs are shiny on one side and dull on the other. The dull side usually has a__23__ on it telling you what’s on the CD； but the shiny side is the important part. It’s shiny so that a laser beam can bounce off the disc and read the information stored on it.
Before CDs were__24__ , music was typically stored on plastic LP (AB) reliable (AC) label (AD) invented (AE) measurement (BC) technically (BD) digitally (BE) protective (CD) remotely (CE) middle (DE) primitive (ABC) player (ABD) converted