Two markers $M_1$ and $M_2$ are arranged with a vertical separation of $h$. A steel ball is dropped at time zero from a point $x$ above $M_1$. The ball arrives at $M_1$ at time $t_1$ and reaches $M_2$ at time $t_2$. Its acceleration remains constant. Which expression gives the acceleration of the ball?
- A$\frac{2h}{t_2^2}$
- B$\frac{2h}{(t_2 + t_1)}$
- C$\frac{2h}{(t_2 - t_1)^2}$
- D$\frac{2h}{(t_2^2 - t_1^2)}$