A Neutron in a Nuclear Reactor Makes an Elastic Head-On

Multiple Choice

A neutron in a nuclear reactor makes an elastic head-on collision with a carbon atom initially at rest. (The mass of the carbon atom is 12 times that of the neutron.) What fraction of the neutron's original kinetic energy is retained by the neutron?

A) 14.4%
B) 28.4%
C) 41.4%
D) 71.6%