A block moves down a smooth inclined plane of inclination 0 . Its velocity on reaching the bottom is $v$. If it slides down a rough inclined plane of same inelination, its velocity on reaching the bottom is $w / n$, where $n$ is a number greater than 1 . The coefficient of friction is given by
(a) $\mu=\tan \theta\left(1-\frac{1}{n^{2}}\right)$
(b) $\mu=\cot \theta\left(1-\frac{1}{n^{2}}\right)$
(c) $\mu=\tan \theta\left(1-\frac{1}{n^{2}}\right)^{1 / 2}$
(d) $\mu=\cot \theta\left(1-\frac{1}{n^{2}}\right)^{1 / 2}$