A charge Q is uniformly distributed over the surface of a non-conducting disc of the radius R. The disc rotates about an axis perpendicular to its planes and passing through its centre with an angular velocity $\mathrm{\omega }$. As a result of this rotation, a magnetic field of induction B is obtained at the centre of the disc. Assume that both the amount of charge placed on the disc and its angular velocity are constant. If the radius of the disc is varying, then the variation of the magnetic induction at the centre of the disc will be represented by which of the following figure?