资料题：A sudden, sharp decline in the value of securities is called（　）。

Security and commodity exchanges are trading post merely provide a place where prospective buyers and sellers can meet and conduct their business.Wall Street, although the best known, is not the only home of exchanges in the United States.There are the cotton exchanges in New Orleans and Chicago; the Mercantile Exchange,which deal in many farm products, in Chicago; and grain exchanges in many of the large cities of the Midwest.Some exchanges, like the Chicago Board of Trade, provide market services for several kinds of products.These trading posts where products may be bought or sold are called commodity exchanges.The security exchanges, on the other band, are meeting places where stocks and bonds are traded.Like the commodity exchanges, they help serve the economic life of the county.But when their operations get out of hand, they may become very dangerous.In 1929, the security exchanges, or stock market, contributed to a crash,a sudden, sharp decline in the value of securities.Many people lost fortunes; many corporations were bankrupted; many workers lost their jobs.The Crash of 1929 has been attributed to many causes, among them wild and unwise speculation by many people and dishonest practices on the part of some businessmen and of some members of the exchanges.