可以作为平屋顶的构造组成的层次有结构层、防水层、（）
A.保温层
B.隔热层
C.隔汽层
D.找坡层
参考答案：A, B, C, D
31. We should think of ways to stop the waste from the factory _____ our living environment.
A．being polluted
B．polluting
C．from being polluted
D．Polluted
晚清四大藏书楼
