A mixture consists of two radioactive materials $A_1$ and $A_2$ with half lives of $20\; s$ and $10\;s$ respectively. Initially the mixture has $40 \;g$ of $ A_1$ and $160\; g$ of $A_2$. The amount of the two in the mixture will become equal after :

Question

( A ) 40 s
( B ) 60 s
( C ) 80 s
( D ) 20 s