--- 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

气缸的排量等于（）。

A.气缸的总容积

B.气缸的工作容积

C.气缸燃烧室容积

D.气缸行程