HSK 7-9
千钧一发
qiānjūn-yífà(lit.) a thousand pounds hangs by a thread (idiom); (fig.) (at the) critical; perilous (moment)
消防员在千钧一发时赶到。
Xiāofángyuán zài qiānjūnyífà shí gǎndào.
The firefighters arrived at the critical moment.
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Appears across levels (7-9) in the HSK 3.0 word list.