具有强迫换流的变流器主要用于直流斩波调压及无源逆变等，故大多采用晶闸管作开关器件
参考答案：错
-Do you know him well?
-Sure. We __________ friends since ten years ago.[ ]
A. were
B. have been
C. have become
D. have made
周围神经损伤后，从神经纤维长入终末器官到效应器官出现生理功能，一般约需（）
A.1周
B.2周
C.3周
D.4周
E.5周
