Once

once ______, this power station will supply all the neighboring towns and villages with electricity.

it being completed

it completed

completed

it completes

答案及解析：本题考查省略句C选项，句意：这座电站一旦建成竣工，就将向周围城乡供电。once在这里是连词，意为“一旦…就…”，后面省略了it is。A，选项，没有省略it，使用排除法，故排除。B，选项，没有省略it，使用排除法，故排除。D，选项，没有省略it，使用排除法，故排除。故正确答案为C选项。