Actually,you can be _________ you want to be,a scientist,a doctor,an astronaut or a manager so long as you set your mind to it.

A．whatever　

B．whoever

C．whomever　

D．whichever 

属于调节斩波器输出电压平均值的方法是（）。

A.定频调宽法；

B.定宽调频法；

C.调频调宽法；

D.上述三种均是。