The price of West Texas Intermediate crude oil had been rising steadily this week as Tropical Storm Gustav gathered strength.

随着热带风暴古斯塔夫不断积蓄力量，西得克萨斯中质油的价格也在本周稳步攀升。

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Oil prices fell to their lowest level in 12 years; futures of West Texas intermediate crude closed at $31.41 a barrel, down 5.3 percent.

石油价格跌至12年来的最低水平，西德克萨斯中质原油（West Texas Intermediate）期货报收每桶31.41美元，下挫了5.3%。

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The ETF purports to track the price of West Texas intermediate crude oil.

FORBES: How To Avoid ETF Pitfalls

The price spread between Brent crude oil, the European benchmark and West Texas Intermediate crude oil, the U.S. benchmark, widened sharply because of the loss of Libyan crude oil production.

FORBES: 'QE3' Would Be The 'Most Bullish Factor' For Commodities -- Panel

As you can see in the chart below, over the past 5-, 15- and 30-year cycles, West Texas Intermediate crude oil prices head higher in March, and have generally continued to climb through September.

FORBES: The Case For Higher Crude Prices