A thin-walled metal spherical shell of radius a has a charge qa. Concentric with it is a thin-walled metal spherical shell of radius b > a and charge qb. Find the electric field at points a distance r from the common center, where

(a) r < a,

(b) a < r < b, and

(c) r > b.

(d) Discuss the criterion you would use to determine how the charges are distributed on the inner and outer surfaces of the shells.