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、脉动直流电压
D、可调直流电压
参考答案：D
Father asked Edison ________ doing that for. [ ]
A. how is he
B. what he was
C. why was he
D. where he is
1/50mm游标卡尺在公制卡尺中是一种精密的卡尺，它能测到的精密度是（）mm。
A．0.01
B．0.02
C．0.05
