U.S. stocks are falling in midday trading, extending losses from last week.
The price of crude oil continued to drop Monday, and Russia's ruble plunged to a record low against the dollar.
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The Dow Jones industrial average fell 156 points, or 0.9 percent, to 17,125 as of noon Eastern time.
The Standard & Poor's 500 fell 18 points, or 0.9 percent, to 1,984. The Nasdaq composite lost 58 points, or 1.3 percent, to 4,595.
Russia's ruble plunged 10 percent to 63.96 to the dollar. The ruble started the year at 32.85 to the dollar.
The falling price of oil, which is the chief source of Russian exports and tax revenue, has weighed heavily on the currency.
The price of U.S. benchmark crude oil sank $1.48 to $56.32 a barrel.