The mountains form a natural between the two countries so they have no trade relations in

问题补充：

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

答案：

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。