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If you say there's no knowing, you mean it is impossible to be certain about something.
无从知晓;难以预料
She's very unpredictable, so there's no knowing how she'll react to the news.
她这个人很难预料，所以不知道她会对这个消息作何反应。
There's no saying what they might be capable of achieving.
他们能取得什么样的成就是很难说的。