The amplitude of a damped oscillator decreases to $0.9$ times its original magnitude in $5s$. In another $10\;s$ it will decrease to α times its original magnitude, where $\alpha$ equals :

Question

$\begin{array}{1 1} (1)\;0.7 \\ (2)\;0.81 \\ (3)\;0.729 \\(4)\;0.6 \end {array}$