一个10W的功率信号，经过一个6dB衰减器之后，功率为（）。
A.10W
B.4W
C.5W
D.2.5W
参考答案：D
The manager_____ the workers how to improve the program since 9 a.m.
A．has told
B．is telling
C．has been telling
D．will have told
有两个同频率的正弦交流电流i1＝Im1sin（ωt＋120°）Ai2＝Im2sin（ωt－120°）A问：哪个电流超前？超前多少度？
