HSK 7-9
艰巨
jiānjù形
(of a task, mission or responsibility) extremely difficult; challenging; demanding
重建古桥是一项艰巨工程。
Chóng jiàn gǔ qiáo shì yí xiàng jiān jù gōng chéng.
Rebuilding the old bridge is a formidable undertaking.
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Appears across levels (7-9) in the HSK 3.0 word list.