in vain的具体用法

　　例句：

　　She searched in vain for her passport.

　　她翻找自己的护照，但没找着。

　　It became obvious that all her complaints were in vain.

　　很明显她所有的抱怨都是白费口舌。

　　A seemingly endless line of trucks waits in vain to load up.

　　一眼望不到头的一长列卡车眼巴巴地等着装货。