A smooth block is released at rest on a `45^(@)` 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
A. `mu_(k)=1-(1)/(n^(2)`
B. `mu_(k) =sqrt((1-(1)/(n^(2)))`
C. `mu_(k) = (1)/(1 -n^(2))`
D. `mu_(k)=sqrt((1)/(1-n^(2))`
