Two parallel rail tracks run north south. Train A moves north with a speed of 54 km/h and train B moves south with a speed of 90 km/h.
The velocity of a monkey running on the roof of the train A against its motion (with a velocity of 18 km/ h with respect to the train A) as observed by a man standing on the ground is
(a) 33 km/h
(b) 36 km/h
(c) 99 km/h
(d) zero
