Discretionary fiscal policy can be defined as:（）

A. a situation in which total government spending exceeds total government revenue during a specific time period, usually one year. 

B. a change in laws or appropriation levels that alters government revenues and/or expenditures. 

C. a situation in which total government spending is less than total government revenue during a time period, usually one year.

目前空调工程中，下面常用的保温材料中，（）只能用于保热管道。

A.岩棉

B.玻璃棉

C.聚氨酯

D.聚乙烯