I _______ my homework, I guess I can't join you. [ ]

A. don't finish

B. didn't finish

C. haven't finished

D. won't finish

把直流电源中恒定的电压变换成（）的装置称为直流斩波器。

A.交流电压；

B.可调交流电压；

C.可调直流电压；

D.脉动方波直流电压。