U.S. stocks opened flat on Thursday as investors found few reasons to buy following a sharp rally in the previous session and amid signs of weakness in Europe and ongoing uncertainty over Ukraine, Reuters reported. The Dow Jones industrial average rose 19.52 points or 0.12 percent, to 16,671.32, the S&P 500 gained 3.12 points or 0.16 percent, to 1,949.84 and the Nasdaq Composite added 5.09 points or 0.11 percent, to 4,439.22.