107、The main hardcopy output devices are printers and plotters.A printer can print characters.symbols, and usually graphics on paper, Printers are categorized according to whether or not the image produced is formed by physical contact of the print mechanism with the paper.Impact printers make contact; nonimpact printers do not.Impact printers usually form images by pressing an inked ribbon against the paper with a hammer-like mechanism.A letter-quality printer like a typewriter, presses fully formed characters against the ribbon.A dot- matrix printer uses separate pin-like hammers to form characters out of dot patterns.Letter quality printers produce higher quality images than do dot- matrix printers and are generally used for business letters, important memos, and reports.However, letter-quality printers are slower and noisier than dot-matrix printers, and they cannot produce graphics.Common types of impact letter-quality printers are the daisy wheel printer, the thimble printer, and the ball printer.Dot-matrix printer, because they do not print fully formed characters, are more flexible and can be used to print graphics in black or in color. The high-volume output requirements of most large computer installations cannot be satisfied by dot-matrix or letter-quality printers.Large computer systems often use special high-speed printers, called line printers that print a whole line at a time.Three common types of line printers are drum printers, band or belt printers, and chain printers.(2)、How many types of impact letter-quality printers are mentioned in this paragraph?