A cannon of mass 5m (including a shell of mass m) is at rest on a smooth horizontal ground, fires the shell with its barrel at an angle θ with the horizontal at a velocity u relative to cannon. Find the horizontal distance of the point where shell strikes the ground from the initial position of the cannon.
A. 16u²sin2θ/9g
B. u²sin2θ/5g
C. 3u²sin2θ/5g
D. 8u²sin2θ/5g