HSK 7-9
不得而知
bùdé’érzhīunknown; unable to find out
事故的真正原因尚不得而知。
Shì gù de zhēn zhèng yuán yīn shàng bù dé ér zhī.
The true cause of the accident remains unknown.
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Appears across levels (7-9) in the HSK 3.0 word list.