The diagram shows a particle X, with kinetic energy $E_k$, just before it collides with a particle Y that is at rest. The two particles have equal mass. Following the collision, X and Y continue moving together as one larger particle. How much kinetic energy is lost in the collision?
- A$0$
- B$\frac{E_k}{4}$
- C$\frac{E_k}{2}$
- D$\frac{3E_k}{4}$