The current density J inside a long, solid, cylindrical wire of radius a = 3.1 mm is in the direction of the central axis, and its magnitude varies linearly with radial distance r from the axis according to J = J0r/a, where J0 = 310 A/m2. Find the magnitude of the magnetic field at

(a) r = 0,

(b) r = a/2 and

(c) r = a.