HSK 7-9
恍然大悟
huǎngrán-dàwù(idiom) to suddenly realize; to suddenly see the light
看到钥匙编号他才恍然大悟。
Kàn dào yào shi biān hào tā cái huǎng rán dà wù.
The number on the key suddenly made him understand.
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Appears across levels (7-9) in the HSK 3.0 word list.