A spherical conductor of radius 'a' with charge 'q' is placed concentrically inside an uncharged and unearthed spherical conducting shell of inner and outer radii r1and r2 respectively. Taking potential to be zero at infinity, the potential at any point P within the shell (r1 < r < r2) will be
`q/(4 pi epsi_0 r)`
`q/(4 pi epsi_0 a)`
`q/(4 pi epsi_0 r_2)`
`q/(4 pi epsi_0 r_1)`