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)$\frac{t_1-t_2}{2}$

(2)$\frac{t_1{t}_2}{t_2-t_1}$

(3) $\frac{t_1{t}_2}{t_2+t_1}$

(4) $t_1-t_2$