No one has （）been able to know who she is.

道理很简单.no one 是否定词.yet通常与否定词连用----- no / not / none -- - - yet.
明白了吗?
再问： 那其他三个呢？
再答： still ----he hasn't come yet == he still hasn't come (注意still的位置) yet -----he hasn't come yet (否定词not - -- - -yet) already / just -----通常用于肯定句-----he has just / already come ok了吗？
再问： 就是 yet 在 否定之后， still 在之前、呗
再答： ok. 要记住哦。very useful.