He let me write down his instruction--- sure that I would kn

在 I would know what was ___ after he went away。这个句子中,
what 引导一个宾语从句,作 would know 的宾语,
在这个宾从中,what 是主语,它是物,不可能发出do的动作,而与do是被动关系,
因此第二空要用被动结构to be done,故答案选B
句意:他让我写下他的指示,以确保他离开之后,我知道什么事情该做。