题目

如图所示，真空中等量异种点电荷放置在M、N两点，在MN的连线上有对称点a、c，MN连线的中垂线上有对称点b、d，则下列说法正确的是（  ）                                                                                                                                                                                          　   A．  a点场强与c点场强一定相同 　   B．  a点电势一定小于c点电势 　   C．  负电荷在c点电势能一定大于在a点电势能 　   D．  正电荷从d点移到b点电场力做正功                                                                                                                                     答案：A

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