单项选择题

On Monday, October 19, 1987, a wave of selling triggered widespread price declines in stock markets from New York to Australia. On that day, now infamous as "Black Monday", over 600 million shares were traded on the New York Stock Exchange. The Dow Jones Industrial Average of the prices of 30 stocks of major US companies lost 22.6 percent of its value on that memorable day, plunging 508 points in the panicked rush to sell.
What is the stock market, and how is it affected by the forces of supply and demand The stock market is the means through which previously issued corporate stocks, shares of ownership in a corporation, are traded. Stock exchanges are organizations whose members act as intermediaries to buy and sell stocks for their clients. About 80 percent of all stock trading in the United States takes place at the New York Stock Exchange. There are other stock exchanges in the United States as well as in Paris, London, Sydney and Tokyo.
How are stock prices determined The answer, as you might expect, is by supply and demand. However, the forces influencing the prices of corporate stocks are quite different from those influencing the prices of goods and services. People and organizations that buy and hold stock do so for the incomes they hope to earn. The incomes depend on dividends paid to stock holders, changes in the price of stock over time, and the expected return on alternative investments.
On any given day in the stock market, there are orders to buy and orders to sell. The orders to buy constitute the quantity of a stock demanded at the current (or anticipated) price per share, while the orders to sell constitute the quantity supplied at that price. The chief influence on both the supply of and demand for stocks is the income potential of holding the stock com pared to the income potential of holding alternative assets such as bonds, other types of securities, or real property like buildings and land.
On the New York Stock Exchange, trading in all stocks is continuous. A specialist is as signed to oversee trading in each stock. This specialist is a "broker’s broker" who tries to adjust the price of the stock so that quantity demanded equals quantity supplied. However, the specialist is also allowed to purchase the stock to hold as a personal investment if no buyer can be found. In this way the specialist can exert some influence on the supply of and demand for stocks, and will do so if it’s profitable.
On October 19, 1987, there were hardly any buy orders, and the markets were flooded with sell orders. Because of the tremendous surplus of stocks at the prevailing prices, specialists and call clerks lowered prices until quantity demanded equaled quantity supplied. When Black Mon day finally reeled to a close, many a portfolio had lost over a fifth of the value it had the day be fore.
By the word "intermediaries" (line 4, paragraph 2), the author means ______.