采用PWM调速技术，直流电动机电枢两端得到的电压u(t)是一串( )
A．三角波
B．方波脉冲
C．锯齿波
D．正弦波
参考答案：B
Can you imagine John _____ his work ahead of time though he had three days off? [ ]
A. finish
B. finishing
C. to finish
D. to have finished
从信息研究的横断面看，出现过哪些理论？
