$A$ thin non-conducting disc of radius $R$ is rotating clockwise (see figure) with an angular velocity $\omega$ about its central axis,which is perpendicular to its plane. Both its surfaces carry positive charges of uniform surface density. Half the disc is in a region of a uniform,unidirectional magnetic field $B$ parallel to the plane of the disc,as shown. Then,

• A
  The net torque on the disc is zero.
• B
  The net torque vector on the disc is directed leftwards.
• C
  The net torque vector on the disc is directed rightwards.
• D
  The net torque vector on the disc is parallel to $B$.