Particle One of Mass M Initially Moves to the Right

Multiple Choice

Particle one of mass m initially moves to the right at v.Particle two of mass 4m is initially at rest.After they undergo an elastic collision,particle one moves directly down.What is the final speed of particle one?

A) 0.316 v
B) 0.775 v
C) 0.200 v
D) 0.800 v
E) 0.707 v