Preeti reached the metro station and found that the escalator was not working. She walked up the stationary escalator in time $t_1$. On other days,if she remains stationary on the moving escalator,then the escalator takes her up in time $t_2$. The time taken by her to walk up on the moving escalator will be

• A
  $\frac{t_1 t_2}{t_2 - t_1}$
• B
  $\frac{t_1 t_2}{t_1 + t_2}$
• C
  $t_1 - t_2$
• D
  $\frac{t_1 + t_2}{2}$