Carbon at 1273 K is contained in a 7-cm-innerdiameter cylinder made of iron whose thickness is 1.2 mm. The concentration of carbon in the iron at the inner surface is 0.5 kg/m³ and the concentration of carbon in the iron at the outer surface is negligible. The diffusion coefficient of carbon through iron is 3 × 10^-11 m²/s. The mass flow rate of carbon by diffusion through the cylinder shell per unit length of the cylinder is
(a) 2.8 × 10^-9 kg/s

(b) 5.4 × 10^-9 kg/s

(c) 8.8 × 10^-9 kg/s

(d) 1.6 × 10^-8 kg/s

(e) 5.2 × 10^-8 kg/s