At a metro station, a girl walks up a stationary escalator in time t_{1}. If she remains stationary on the escalator, then the escalator take her up in time t_{2}. The time taken by her to walk up on the moving escalator will be

(a) (t_{1} + t_{2})/2

(b) t_{1}t_{2}/(t_{2} – t_{1})

(c) t_{1}t_{2}/(t_{2} + t_{1})

(d) t_{1} – t_{2}