A wooden block of mass M rests on a horizontal surface. A bullet of mass m moving in the horizontal direction strikes and gets embedded in it. The combined system covers a distance x on the surface. If the coefficient of friction between wood and the surface is `mu` , the speed of the bullet at the time of striking the block is (where m is mass of the bullet)

A. `sqrt((2Mg)/(mum))`

B. `sqrt((2mumg)/(Mx))`

C. `sqrt(mugx)((M+m)/(m))`

D. `sqrt((2mumx)/(M+m))`