HSK 7-9
乃
nǎi副
to be; thus; so; therefore
沟通乃长期合作的基础。
Gōutōng nǎi chángqī hézuò de jīchǔ.
Communication is the foundation of long-term cooperation.
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Appears across levels (7-9) in the HSK 3.0 word list.