In a Certain Nuclear Reactor, Neutrons Suddenly Collide with Carbon

Multiple Choice

In a certain nuclear reactor, neutrons suddenly collide with carbon nuclei, which have 12 times the mass of neutrons. In a head-on elastic collision with a stationary carbon nucleus, what fraction of
Its initial speed does the neutron have after the collision?

A) 5/6
B) 11/12
C) 1/12
D) 10/13
E) 11/13