The mountains form a natural between the two countries so they have no trade relations in history.
A. exchange
B. bridge
C. barrier
D. fence
网友回答
A. exchange
B. bridge
(答案→)C. barrier
D. fence
解析：这两国之间在历史上之所以没有贸易往来，是因为它们之间有一道天然屏障，难以逾越。如果是fence，则指实实在在的栅栏，难以构成障碍。