A uniform electric field of strength $E$ exists in a region. An electron (charge $–e$, mass $m$) enters a point $A$ with velocity $V \hat{\mathrm{j}}$. It moves through the electric field and exits at point $B$.Then:

 

Four short dipoles, each of dipole moment $p$, are placed at the vertices of a square of side $a$. The direction of the dipole moments are shown in the figure.

Figure shows a charge $Q$ placed at the centre of the open face of a cylinder as shown in the figure. A second charge $q$ is placed at one of the positions $A, B, C$ and $D$, out of which positions $A$ and $D$ are lying on a straight line parallel to the open face of the cylinder. In which position(s) of this second charge, the flux of the electric field through the cylinder remains unchanged?