Two point charges $-q$ and $+q$ are located at points $(0,0,-a)$ and $(0,0,a)$ respectively.The electric potential at a point $(0,0,z)$ where $z > a$ is :

Question

$$\begin {array} {1 1} (1)\;\frac{qa}{4 \pi \in _0 z^2} & \quad (2)\;\frac{q}{4 \pi \in _0 a} \\ (3)\; \frac{2qa}{4 \pi \in _0(z^2-a^2)} & \quad (4)\;\frac{2qa}{4 \pi \in _0(z^2+a^2)} \end {array}$$