火灾自动报警系统的主电源应采用（ ）。（易）

A、消防电源

B、动力电源

C、照明电源

D、直流电流

—I think recycling books and paper is useful.     

—______[ ]

A. I quite agree.            

B. I have no idea.

C. We shouldn't use such things.    

D. That's a great idea.