中性点直接接地系统发生一相接地时，系统中性点仍保持对地电位，

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问题单项选择题

中性点直接接地系统发生一相接地时，系统中性点仍保持对地电位，但故障点的电流不再是系统对地电容电流，而是（）电流。

A.三相短路

B.两相短路

C.单相接地短路

D.不平衡

答案

参考答案：C

填空题

设a，b为空间的两条直线，α，β为空间的两个平面，给出下列命题：

①若a∥α，a∥β，则α∥β；②若a⊥α，α⊥β，则α⊥β；

③若a∥α，b∥α，则a∥b; ④若a⊥α，b⊥α，则a∥b.

上述命题中，所有真命题的序号是________．

单项选择题

Which of the following statements about portfolio risk and diversification is least accurate（）

A. Unsystematic risk can be substantially reduced by diversification.

B. Systematic risk can be eliminated by holding securities in a well-diversified international stock portfolio.

C. Diversification results from combining securities that have less than perfect positive correlation between their returns to reduce portfolio risk.