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 :

Question

( A ) $\frac{t_1t_2}{t_2 - t_1}$
( B ) $t_1 - t_2$
( C ) $\frac{t_1 + t_2}{2}$
( D ) $\frac{t_1t_2}{t_2 + t_1}$