A bar of pure nickel is coupled with a bar of pure iron (interfaced). The diffusion couple is then heated to a temperature of 1000 degrees C. (a) How long will it take for the concentration of nickel to reach 0.1wt%, 1.0um below the interface? (b) How long will it take for the concentration of nickel to reach 0.1wt%, 1.0mm below the interface? (c) What does the comparison of the two answers show?