[This river forms a natural _______ between China and Korea, A.boundary, B.string, C.spot, D.zone]$[A]
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This river forms a natural _______ between China and KoreaA.boundaryB.stringC.spotD.zone
[This river forms a natural _______ between China and Korea, A.boundary, B.string, C.spot, D.zone]$[A]
