The steepest drop in oil prices in more than three years put investors in a selling mood Tuesday, extending a losing streak for the S&P 500 index to a fourth day. Energy stocks led a late-afternoon sell-off on Wall Street after the price of U.S. crude oil plunged 7.1 percent to $55.69 a barrel, the lowest level since December 2017. Oil has now fallen for 12 straight days, driven by worries over rising oil production around the world and weakening demand from developing countries. "You have fears associated with the drop in the price of oil probably moving into the equity market," said Willie Delwiche, investment strategist at Baird. "There's a knee-jerk reaction when you see oil down that it signals economic weakness.