三相交流电、采用三相四线制时，电源的中线不得加装电源保险丝。
参考答案：对
I have a lot of homework today.I won’t go out until ___done.
A．it is
B．it has
C．it will be
D．it has been
对于犯罪分子在不同时空的出现，决定了公安工作的集中性。( )
