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