For reproducing sound, a CD (Compact Disc) audio player uses a

1. A CD (Compact Disc) audio player reproduces sound by reading digital data stored on the disc. 2. The data is encoded as microscopic pits and lands on the disc surface. 3. The player uses a focused laser beam to read this pattern. The laser reflects differently off the pits and lands. 4. A photodiode detects the variations in the reflected light, which are then decoded into the audio signal. 5. Quartz crystals are used for timing, needles are used in record players (not CDs), and barium titanate is a piezoelectric ceramic, none of which is the primary tool for reading a CD.