A block of mass `m` slides down an inclined right angled trough .If the coefficient of friction between block and the trough is `mu_(k)` acceleration of the block down the plane is
A. `(costheta-musintheta)g`
B. `(sintheta-musintheta)g`
C. `(sintheta-mucostheta)g`
D. `(sintheta-sqrt(2)mucostheta)g`