A Solid Metal Sphere of Radius R Is Inside a Metal

Multiple Choice

A solid metal sphere of radius r is inside a metal concentric spherical shell of inner radius 2r and outer radius 3r.A charge is place on the solid metal sphere and it is found that the potential at its surface is V​₁ (at r) and that the potential at the outer surface of the spherical shell is V​₃ (at 3r) .In terms of V​₁ and V​₃,what is the potential at the inner surface of the metal shell (at 2r) ?

A) 3V​₁/2
B) V​₃
C) V​₁
D) 2V​₃/2