A neutron travelling with a velocity `v` and kinetic energy `E` collides perfectly elastically head on with the nucleus of an atom of mass number `A` at rest. The fraction of the total kinetic energy retained by the neutron is
A. `((A-1)/(A+1))^(2)`
B. `((A+1)/(A-1))^(2)`
C. `((A-1)/(A))^(2)`
D. `((A+1)/(A))^(2)`
