The diagram shows the arrangement of three small uniformly charged spheres $A$, $B$ and $C$. The arrows indicate the direction of the electrostatic forces acting between the spheres (for example, the left arrow on sphere $A$ indicates the electrostatic force on sphere $A$ due to sphere $B$). At least two of the spheres are positively charged. Which sphere, if any, could be negatively charged?

 

Figures below show regular hexagons, with charges at the vertices. In which of the following cases the electric field at the centre is not zero?

Two conducting plates $A$ and $B$ each having large surface area $S$ (on one side) are placed parallel to each other. The plate $A$ is given a charge $q$, while the plate $B$ is neutral. Then, the electric field at a point in between the plates is:

The figure shows a charge $q$ placed inside a cavity in an uncharged conductor. Now if an external electric field is switched on