－It's difficult 'for the village children to cross the river for school.

－I think a bridge _________ over the river  [ ]

A. should be built      

B. is being built

C. has been built        

D. was built

操作系统是计算机系统中最重要的（）软件之一。

A.实用

B.应用

C.系统

D.用户