The surface of an iron component is to be hardened by carbon. The diffusion coefficient of carbon in iron at 1000°C is given to be 3 × 10^-11 m²/s. If the penetration depth of carbon in iron is desired to be 1.0 mm, the hardening process must take at least
(a) 1.10 h

(b) 1.47 h

(c) 1.86 h

(d) 2.50 h

(e) 2.95 h