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[单选题]
This river forms a natural _______ between China and Korea.
A.boundary
B.string
C.spot
D.zone
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A.boundary
B.string
C.spot
D.zone
A.错误
B.正确
The Yellow River is the second ______river in China.
A.long
B.longer
C.longest
D.most long
A.正确
B.错误
A.invitation
B.challenge
C.request
D.demand
When he was a boy, Mark Twain dreamed of traveling on river boats to______
A.become a riverboat pilot
B.gather material for his books
C.make money
D.enjoy himself