HSK 7-9
底线
dǐxiàn名
bottom line; the limit of what one is prepared to accept; (sports) baseline; (sewing) under thread; spy; informer; plant
诚实是合作不能突破的底线。
Chéng shí shì hé zuò bù néng tū pò de dǐ xiàn.
Honesty is a bottom line that cooperation must not cross.
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Appears across levels (7-9) in the HSK 3.0 word list.