It is the twenty-first Football World Cup.

这是第二十一届足球世界杯。

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You might have to stay away from low-lying areas for as long as twenty hours after the first warning.

VOA: special.2011.06.07

All of this is Darwinian thinking and, I think, increasingly will be central in importance in the twenty-first century.

这就是达尔文所思考的，我认为，这在二十一世纪会越来越重要。

耶鲁公开课 - 文学理论导论课程节选

The First Bank of the United States was created in 1791 and they had a twenty-year charter which expired in 1811.

美国第一银行于1791年,建立并且它们有20年的许可证,到1811年到期。

耶鲁公开课 - 金融市场课程节选

Twenty-First's Web site ( www.twenty-first.com) has a calculator to help with your decision.

FORBES: Your Portfolio Versus the IRS

All three missions contribute to make a library of the twenty first century the ideal place to discover new books.

FORBES: Why Public Libraries Matter: And How They Can Do More

In the twenty first century, popularizing science is not merely a philanthropic, democratic option, but rather a definite need of our society.

UNESCO: Science Policy and Capacity-Building