A given object takes $n$ times more time to slide down $45^{\circ}$ rough inclined plane as it takes to slide down a perfectly smooth $45^{\circ}$ incline. The coefficient of kinetic friction between the object and the incline is
a. $\frac{1}{2-n^{2}}$
b. $1-\frac{1}{n^{2}}$
c. $\sqrt{1-\frac{1}{n^{2}}}$
d. $\sqrt{\frac{1}{1-n^{2}}}$