A Proton, of Mass M and Speed V, Strikes an Alpha-Particle

Multiple Choice

A proton, of mass m and speed v, strikes an alpha-particle (which consists of 2 protons and 2 neutrons) at rest. If the collision is completely elastic, what speed will the $\alpha$ -particle have after the collision? (Assume the neutron's mass equals the proton's mass.)

A) 5/3 v
B) zero
C) 2/5 v
D) 2/3 v
E) 3/2 v