4、如图所示,长为a的细线AB上均匀地分布着总电量为Q的正电荷。O点为BA延长-|||-线上一点,且到A点的距离也为a。则0点的电场强度大小为:-|||-(A) dfrac (Q)(4pi {varepsilon )_(0)(a)^2}; (B) dfrac (3Q)(4pi {varepsilon )_(0)(a)^2}; E Q-|||-0 a A a B-|||-(C) dfrac (Q)(8pi {varepsilon )_(0)(a)^2}; (D) dfrac (Q)(2pi {varepsilon )_(0)(a)^2}