单项选择题
A.N极B.S极C.N极和S极D.以上均不正确
A.2QH 2QHJHB.2QH 2QJ 2HC.2QH 2Q 2HJD.2QH 2QHJ 2H
A.2DF 2DJ 2F2DF 2DJ 2FB.2DF 2D 2FJC.2DF 2DFJD.2D 2DFJ
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