A cart of mass m travels to the right with speed v on a frictionless track and strikes a stationary cart of mass 2m in a perfectly elastic head-on collision. Which of the following best describes the subsequent motion of the mass-m cart?
a) It continues to the right with speed 31v.
b) It comes to rest immediately after the collision.
c) It reverses direction and moves to the left with speed 31v.
d) It reverses direction and moves to the left with speed v.