A ball of mass $m$, travelling with velocity $v$, strikes a second ball of mass $2m$ that is stationary. The two balls remain joined together. What fraction of the original kinetic energy is lost during the collision?
- A$\frac{1}{9}$
- B$\frac{1}{3}$
- C$\frac{2}{3}$
- D$\frac{8}{9}$