Which one of the following statements is true?

(a) The length of dislocation line per unit volume in a material equals the number of dislocations per unit area.

(b) Metals are hardened by cold working because many of the dislocations have passed right through the grains and are no longer available to facilitate slip.

(c) Plastic deformation of a polycrystalline metal does not increase the density of the metal.

(d) To avoid breaking a material when it is being cold worked, it is sufficient to perform the cold work in compression.

(e) To avoid breaking a material when it is being cold worked, it is necessary to perform the cold work in compression.