电子束焊机应有良好的地线装置，进行绝缘试验时，试验电压应为额定电压的( )倍。
A．1
B．1.2
C．1.4
D．1.5
参考答案：D
My favourite photo _____ by my mother when I was two years old.[ ]
A. takes
B. is taken
C. took
D. was taken
单边带接收机AGC电路的时间常数应满足（）。
A、快充、快放、快恢复
B、慢充、慢放、慢恢复
C、快充、慢放、快恢复
D、快充、快放、慢恢复
