Two identical spheres, each perfectly elastic, have equal mass $m$. They move towards one another at the same speed $v$ across a horizontal surface with no friction. Which statement correctly describes the total kinetic energy of the spheres?
- AThe sum of their kinetic energies before impact is zero.
- BThe sum of their kinetic energies before impact is $\tfrac{1}{2}mv^2$.
- CThe sum of their kinetic energies after impact is zero.
- DThe sum of their kinetic energies after impact is $mv^2$.