futile的名詞形式是什么

　　例句：

　　It would be futile to protest.

　　抗議也無用。

　　It would be futile to argue with him.

　　跟他爭辯是徒勞的。

　　There is no point in continuing this futile fight!

　　持續這場無效的戰斗沒有意義！