HSK 7-9
攻关
gōngguān动
to storm a strategic pass; fig. to tackle a key problem
科研小组集中力量技术攻关。
Kē yán xiǎo zǔ jí zhōng lì liàng jì shù gōng guān.
The research team concentrated its efforts on tackling a key technical problem.
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Appears across levels (7-9) in the HSK 3.0 word list.