U.S. stocks opened lower Tuesday, as investors turned cautious in the wake of the downing of a Russian jet fighter by the Turkish military. The S&P 500 opened 9 points, or 0.4%, lower at 2,077. The Dow Jones Industrial Average lost 69 points, or 0.4%, to 17,713 at the open. The Nasdaq Composite began the day down 25 points, or 0.5%, at 5,077.
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