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_(s)=1-1/(n^(2))`
B. `mu_(s)=sqrt(1-1/(n^(2)))`
C. `mu_(k)=1-1/(n^(2))`
D. `mu_(k)=sqrt(1-1/(n^(2)))`