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:

1. A
   $\frac{t_1{t}_2}{t_2-t_1}$
2. B
   $\frac{t_1{t}_2}{t_1+t_2}$
3. C
   $t_1-t_2$
4. D
   $\frac{t_1+t_2}{2}$