在正常或事故情况下,为保证电气设备可靠地运行,必须在电力系统中某点直接或经特殊装置如消弧线圈、电抗、电阻、击穿熔断器与地作金属连接,称为静电接地。( )
参考答案:错
我国各民族的分布特点是 [ ]
A.大集中小分散
B.各民族平均分布
C.大杂居小聚居
D.一个民族一个聚居地
My mother _________ a teacher for ten years. [ ]
A. has has
B. has been
C. have had
D. has had