21的英文怎么写序数词

　　The battle would start the next morning — Sunday, July twenty-first.

　　这场战役就要在第二天早晨，即7月21日星期日打响。

　　But in the twenty-first century, a very different story is unfolding.

　　但在21世纪，呈现在大家面前的又是另外一种故事。

　　I have no doubt that America can compete-and succeed-in the twenty-first century.

　　毋庸置疑，我认为美国有实力参与21世纪的竞争并取得成功。