A smooth block is released at rest on a $45^\circ$ incline and then slides a distance d. The time taken to slide is n times as much to slide on rough incline than on a smooth incline. The coefficient of friction is\\
(1) $\mu_\mathrm{k} = 1 - \frac{1}{\mathrm{n}^2}$\\
(2) $\mu_\mathrm{k} = \sqrt{1 - \frac{1}{\mathrm{n}^2}}$\\
(3) $\mu_s = 1 - \frac{1}{n^2}$\\
(4) $\mu_s = \sqrt{1 - \frac{1}{n^2}}$