The following graph describes the height (in meters) of a rider on a ferris wheel aftertime t (in minutes). How long does it take for the Ferris wheel to go through onerotation?

h(t) 120- 110- 100- 90- 80- 70- 60- 50- 40- 30- 20+ 10- 10 15 10- 20 25 -20- a) 20 min ( b) 30 min O c) 45 min O d) 15 min