听力原文： What are zebra stripes on the tomato soup cans and potato chip bags? They are special black and white vertical lines. These black and white stripes can be read by an optical scanner, or computer. The 'scanner', which reads the lines of code, is a small laser beam of light. When the light ray is broken by the black stripes, the computer reads the information about the product. The code is called the Universal Product Code (UPC). It is designed to add speed and efficiency to stores by using computers. It can do this in several ways. First, it is not necessary for workers to put individual prices on items. Errors in prices are eliminated, or done away with. Also, the time spent checking out is reduced. UPC gives an itemized receipt to the customer with all the items, prices, and totals. The store clerk does not need to spend time checking the price on the item and punching it into cash register. This UPC computerized system will probably save as much as 45 percent of the checkout clerk's time. What do the stripes mean? How does the computer interpret the stripes? The number printed below the vertical bars identifies the manufacturer and the manufacturer's item. As many as 10 trillion individual machinereadable numbers can be used with this system. Codes are scanned by ______.