$.$A solid, uncharged conducting sphere of radius $3$a contains a hollowed spherical region of radius a$.$ A point charge $+$Q is placed at the common center of the spheres. Taking V $=0$ as r approaches infinity, the potential at position r $=2$ a from the center of the spheres is: $($A$)0($B$)23$ kQ a $($C$)$ kQ $3$a $($D$)$ kQ a