pU.S. stocks fell at the open on Thursday as investors weighed the previous day's comments from Fed Chair Janet Yellen, who raised the possibility of an earlier-than-expected hike in interest rates. The Dow Jones industrial average fell 22 points or 0.13 percent, to 16,200.6. The S&P 500 was down 1.5 points or 0.08 percent, to 1,859.31. The Nasdaq Composite fell 3.4 points or 0.08 percent, to 4,304.252.