A:the past one
B:the past
C:which of the past
D:those of the past
如图所示的电路,开关s闭合时,发现L1、L2都不亮,用电压表逐段测量,结果是Uad="6" V,Uab =0,Ubc =0,Ucd ="6" V,则该电路的故障可能是()
若一元二次方程x2+2x+m=0有实数解,则m的取值范围是( )