独立避雷针接地电阻应不大于（）Ω。
A、4
B、10
C、30
参考答案：B
A．E=，m=0
B．E=，m=
C．E=，m=0
D．E=，m=
— Has Sam finished his homework today?
— I have no idea. He ________ it this morning.
A．did
B．was doing
C．has done
D．had done
，m=0
，m=
，m=0
，m=