Stocks plunged Wednesday on deepening worries over a slowdown in the global economy. The Dow closed down 800 points, or about 3%. Investors have been whipsawed in recent days by mixed signals emerging from the Trump administration about tariffs and the escalating trade war with China. The jitters were exacerbated amid worrisome economic data from two big countries. Germany posted negative growth in the latest quarter, and China’s growth in industrial output fell to a 17-year low.