A Sample of radioactive material $A$, that has an activity of $10 \mathrm{ mCi}$ ($1 \mathrm{ Ci} = 3.7 \times 10 ^ { 10 }$ decays $\mathrm { s } ^ { - 1 }$), has twice the number of nuclei as another sample of a different radioactive material $B$ which has an activity of $20 \mathrm{ mCi}$. The correct choices for half-lives of $A$ and $B$ would then be, respectively:\\
(1) 5 days and 10 days\\
(2) 10 days and 40 days\\
(3) 20 days and 10 days\\
(4) 20 days and 5 days