1.在点电荷 +9 的电场中,若取图 2-1 中P点处为电势零点,则M点的电势为 () 。-|||-(A) dfrac (9)(4pi {varepsilon )_(0)a} (B) dfrac (9)(8pi {varepsilon )_(0)a} (C) dfrac (-9)(8pi {varepsilon )_(0)a} (D) dfrac (-q)(4pi {varepsilon )_(0)a}-|||-Q-|||-R1-|||-r P-|||-R2-|||-+q P M-|||-a a-|||-照 =-1 图 2-2