Two small charged metal sphere A and B are situated in a vaccum. The distance between the centres of the spheres is 12.0 cm, as shown in the fig.
The charge on each sphere may be assumed to be a point charge at the centre of the sphere. Point P is a movable point that lies on the line joining the centres of the sphere and is distance x fromt he centre of sphere A. The variation with distance x fo the electric field strength E at point P as shown in the figure.

State the evidence provided by in the fig for the statement that the spheres are conductors, the charges on the spheres are either both positive or both negative.