Two charges, each equal to $q$, are kept at $x=-a$ and $x=a$ on the $x$ -axis. A particle of mass $m$ and charge $q_0=\frac{q}{2}$ is placed at the origin. If charge $q_0$ is given a small displacement $y$ ($y\ll a$) along the $y$ -axis, the net force acting on the particle is proportional to:

1. A
   -y
2. B
   1/y
3. C
   -1/y
4. D
   y