4.在相距为2R的点电荷 +9 与 -9 的电场中,把点电荷 +0 从O点沿OCD移到D点(如图)-|||-,则电场力所做的功和 +0 电位能的增量分别为:[A]-|||-(A) dfrac (qQ)(6pi {varepsilon )_(0)R} , -dfrac (qQ)(6pi {varepsilon )_(0)R} . C-|||-R-|||-+Q-|||-(B) dfrac (qQ)(4pi {varepsilon )_(0)R} , -dfrac (qQ)(4pi {varepsilon )_(0)R} 一 +q 0 -q D --|||-(C) -dfrac (qQ)(4pi {varepsilon )_(0)R} , dfrac (qQ)(4pi {varepsilon )_(0)R} - (D) -dfrac (qQ)(6pi {varepsilon )_(0)R} , dfrac (qQ)(6pi {varepsilon )_(0)R} 一