Two long, charged, thin-walled, concentric cylindrical shells have radii of ri = 3.0 cm and r0 = 6.0 cm. The charge per unit length is 5.0×10−6 C/m on the inner shell and −7.0×10−6 C/m on the outer shell. What is the electric field at a radius r = 8.0 cm ? Take the field to be positive if radially outward and negative if radially inward. 2.3×106 N/C. 4.5×105 N/C. −4.5×105 N/C. −2.3×106 N/C