---It’s difficult for the villagers to cross the river.-I think a bridge over the river

问题补充：

---It’s difficult for the villagers to cross the river.

---I think a bridge over the river.

A. should be built

B. is being built

C. has been built

D. was built

答案：

(答案→)A. should be built

B. is being built

C. has been built

D. was built

解析：本题考查的是情态动词的被动语态，其结构为主语+情态动词+be+动词过去分词，故选答案为A