单项选择题
A.20m B.40m C.60m D.80m
A.变压器的机壳 B.低压侧的交流零线 C.低压侧的火线 D.与变压器相连的电力电缆的金属外护层
A.100mm*10mm B.20mm*2mm C.30mm*3mm D.40mm*4mm