B. It's no use doing STH.

表示“做某事是没有用处的”。

It's no use saying: "We are doing our best." You have got to succeed in doing what is necessary.

光是尽力而为是不够的，为了争取成功，势必还得克服所有困难险阻才行。

It's no good (use) doing that.

那样做没好处(没用)。