Spheres $X$ and $Y$ make up an isolated system. Sphere $Y$ has a greater mass than sphere $X$. At the start, sphere $Y$ is at rest. Sphere $X$ undergoes an elastic collision with sphere $Y$. Before the collision, the speed of sphere $X$ is $u$. Which statement must be correct?
- ASphere $X$ rebounds with a speed that is greater than $u$, and sphere $Y$ moves off with a speed that is less than $u$.
- BSphere $X$ rebounds with a speed that is less than $u$, and sphere $Y$ moves off with a speed that is less than $u$.
- CSphere $X$ rebounds with speed $u$, and sphere $Y$ remains stationary.
- DSphere $X$ remains stationary, and sphere $Y$ moves off with a speed that is less than $u$.