— Are you still busy?  

— Yes, I _____ my work, and it won't take long.  [ ]

A. just finish  

B. am just finishing  

C. have just finished  

D. am just going to

开关电源稳定输出电压的原理可以直观理解为是通过控制滤波电容的（）来实现的。

A.实际容量

B.充、放电时间

C.电压

D.电流