问题
选择题
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
答案：C
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。