preeti reached the metro station and find out that escalator was not working . She walked up the stationary escalator in time t1. on the other hand if she remains stationay on moving escalator, then escalator took her up in time t2. The time taken by her to walk up on the moving escalator will be  :

(A) t1t2/ t1+t2

(B) t1-t2

(C) t1 +t2/2

(D) t1t2/t1 -t2