4.如图4题图中所示为一沿x轴放置的"无限长"分段均匀带电直线,电荷线密度分-|||-别为 +lambda (xlt 0) 和 -lambda (xgt 0), 则Oxy坐标平面上点(0,a)处的场强E为[ ].-|||-(A)0. (B) dfrac (lambda )(2pi {varepsilon )_(0)a}. (C) dfrac (lambda )(4pi {varepsilon )_(0)a}. (D) dfrac (lambda )(4pi {varepsilon )_(0)a}(i+j).-|||-y-|||-(0 ,a )-|||-+λ -λ-|||------