A four-day surge in the stock market came to an end Friday as falling commodity prices brought down energy and mining companies. Signs of a slowing economy rattled commodity markets. The price of crude oil dropped 2 percent to $91 a barrel as weak US economic reports followed forecasts for weaker oil demand. Gold plunged $64 to $1,501 an ounce, reaching its lowest level since July 2011. One trigger for the plunge was a government report that US wholesale prices fell in March. The Dow and S&P 500 finished the week with strong gains. This week, Bank of America, Google, and other big names turn in their quarterly results.