3.真空中一半径为R的球面均匀带电Q,在球心O处有一带电量为q的点电荷。-|||-设无穷远处为电势零点,则在球内离球心O距离为r的P点处的电势为 () 。-|||-(A) dfrac (q)(4pi {varepsilon )_(0)r} (B) dfrac (1)(4pi {varepsilon )_(0)}(dfrac (q)(r)+dfrac (Q)(R))-|||-(C) dfrac (q+Q)(4pi {varepsilon )^or} (D) dfrac (1)(4pi {varepsilon )_(0)}(dfrac (q)(r)+dfrac (Q-q)(R))