-3Q b X= - x =d x a A solid conducting metal sphere of radius b and total charge -3Q has an empty spherical cavity of radius a. An unknown point charge q is placed at the centre of the spherical cavity. The electric field at x = +d is given by E(x = d) = - Q ZnEd? L. What is the charge density on the surface of the metal sphere (r = b)?Q A 41tb 2 30 B 47tb 2 50 C 41 b2 Q D 4762 Q E 21b2