Radioisotope $X$ undergoes decay to form the stable isotope $Y$. The graph illustrates how the mass of $Y$ in a sample changes as time passes. Which time interval corresponds to the half-life of $X$?
- A$t_2 - t_1$
- B$t_3 - t_2$
- C$t_2$
- D$\tfrac{1}{2} t_3$