In the figure below, a nonconducting spherical shell of inner radius a = 1.60 cm and outer radius b = 2.10 cm has (within its thickness) a positive volume charge density ? = A/r, where A is a constant and r is the distance from the center of the shell. In addition, a small ball of charge q = 20.0 fC is located at the center. What value should A have if the electric field in the shell (a r b) is to be uniform?