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Stocks tumbled, battering the Dow after reports of sluggish US growth added to investor worries about a weaker global economy. The slump follows the index’s worst January since 2009. The market stumbled from the opening, and quickly turned into a slide as discouraging news on manufacturing, auto sales, and construction spending poured in. Some investors took the fall in stride, considering it a needed recalibration after the record highs at the end of 2014. Staffing firm Robert Half International fell the most in the S&P. Among big automakers, an icy January slowed purchases. Pfizer was among the few stocks to eke out gains on the day.