A radioactive sample `S_(1)` having an acitivity of `5 mu Ci` has twice the number of nuclei as another sample `S_(2)` which has a activity of `10 mu Ci`. The half lives of `S_(1)` and `S_(2)` can be
A. `20` years and `5` years, respectively
B. `20` years and `10` years, respectively
C. `10` years each
D. `5` years each