－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

下列材料中（）不属活性混合材料。

A.火山灰

B.磨细石灰石

C.烧粘土粉

D.粒化高炉矿