A smooth sphere of mass M moving with velocity u directly collides elastically with another sphere of mass m at rest. After collision their final velocities are V and v respectively. The value of v is
A. `(2uM)/(m)`
B. `(2um)/(M)`
C. `(2u)/(1+(m)/(M))`
D. `(2u)/(1+(M)/(m))`
