CNN Money reports:

The Dow plummeted 531 points, concluding its worst week since 2011. Around the world, major indexes also fell for the week and most are negative for the month.

Three big factors are driving this week’s worries:

1. Concerns are growing that China’s economy is slowing down faster than its government has said.
2. Investors had been preparing for the U.S. Federal Reserve to raise its benchmark interest rate in September. However, the central bank has been sending mixed signals. That’s dredging out the market’s worst enemy: uncertainty.
3. Oil prices dropped below the key $40 level for the first time since 2009.

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