The electron mobility in N-type germanium is 3900 ${\mathrm{cm}}^{2}$/v-s and its conductivity is 6.24 mho/cm, then impurity concentration will be if the effect of cotters is negligible
(a)                   (b) ${10}^{13}/{\mathrm{cm}}^{3}$
(c) ${10}^{12}/{\mathrm{cm}}^{3}$                   (d) ${10}^{16}/{\mathrm{cm}}^{3}$