【正确答案】Stock exchanges are formal organizations. They are made up of members who use the exchange facilities and systems to exchange or trade listed stocks. These exchanges are physical locations where members assemble to trade. Stocks that are traded on an exchange are said to be listed stocks. That is, these stocks are individually approved for trading on the exchange by the exchange. To be listed, a company must apply and satisfy requirements established by the exchange for minimum capitalization, shareholder equity, average closing share price, and other 　　criteria. Even after being listed, exchanges may delist a company's stock if it no longer meets the exchange requirements. 　　To have the right to trade securities or make markets on an exchange floor, firms or individuals must become a member of the exchange, which is accomplished by buying a seat on the exchange. The number of seals is fixed by the exchange and the cost of a seat is determined by supply and demand of those who want to sell or buy seats. In early 2001, there was 1, 366 seats on the NYSE, and the cost of a seat were $2 million. 　　Stock exchanges perform important roles in national economies. Most important, they encourage investment by providing places for buyers and sellers to trade securities. This investment, in turn, enables corporations to obtain funds to expand their businesses.