HSK 7-9
界线
jièxiàn名
limits; bounds; dividing line
河流成为两国的自然界线。
Hé liú chéng wéi liǎng guó de zì rán jiè xiàn.
The river forms a natural boundary between the two countries.
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Appears across levels (7-9) in the HSK 3.0 word list.