Stocks opened mixed as traders braced for additional volatility in a week that’s already been bumpy. The Dow Jones Industrial Average slipped 0.1%, 22 points, to start trading near 27,880, while the S&P 500 was little-changed, opening near 3,360. The Nasdaq Composite was about 59 points higher, 0.5%, trading near 10,969. Friday marks quadruple witching, which refers to the day when various types of financial-asset options expire, traditionally leading to intraday volatility. Shares of United States Steel Corporation dipped despite reporting better-than-expected earnings.