回複：breaking news Nasdaq cancels all the trades during the big dr

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Many professional investors and traders use computer program trading to buy and sell orders for large blocks of stocks. The programs use mathematical models that are designed to give a trader the best possible price on shares.

The programs are often set up in advance and allow computers to react instantly to moves in the market. When a stock index drops by a big amount, for example, computers can unleash a torrent of sell orders across the market. They move so fast that prices, and in turn indexes, can plunge at the fast pace seen Thursday.