The count rate of a Geiger- Muller counter for the radiation of a radioactive material of half life of 30 minutes decreases to 5 ${\mathrm{s}}^{-1}$ after 2 hours. The initial count rate was

(a) 25 ${\mathrm{s}}^{-1}$               (b) 80 ${\mathrm{s}}^{-1}$
(c) 625 ${\mathrm{s}}^{-1}$              (d) 20 ${\mathrm{s}}^{-1}$