The stock market had its worst drop in nearly a month as investors worried about more signs of weakness in China’s economy. The Dow Jones industrial average dropped 312 points, or 1.9 percent, to 16,001 Monday. The Standard & Poor’s 500 index sank 49 points, or 2.6 percent, to 1,881. The Nasdaq composite lost 142 points, or 3 percent, to 4,543. Biotechnology stocks, until recently a high-flying sector, continued to drop sharply. Mining, energy and basic materials stocks also fell hard. Tumbling prices for oil, copper and other commodities have hurt profits at those companies.