消弧线圏接地系统，在正常运行情况下，中性点的长时间电压位移不应超过系统 标称相电压的（）。

单项选择题在模拟法描绘静电场实验中（ ）A.电极间水深不一致，导致电力线为椭圆B.若两不同电势值的等势线不同心，则电源电压有较大波动C.实验发现描绘的等势线误差较大，可能原因是水的导电率不均匀

单项选择题One reason we need government, even in a market economy, is that ______. A、there is insufficient market power in the absence of government B、property rights are too strong in the absence of government C、the invisible hand is not perfect D、Both a and b are correct.