A Sample of radioactive material $A,$ that has an activity of $10 m \mathrm{Ci} \left(1 Ci=3.7\times {10}^{10} \mathrm{d}\mathrm{e}\mathrm{c}\mathrm{a}\mathrm{y}\mathrm{s} {\mathrm{s}}^{-1}\right),$ has twice the number of nuclei as another sample of a different radioactive material $B$ which has an activity of $20 m \mathrm{Ci}$. The correct choices for half-lives of $A$ and $B$ would then be, respectively: