Sean is spending the day at the water park. He paid to enter the park and can use any water ride an unlimited number of times throughout the day. His favorite ride is the Twisty River. If Sean's total utility begins to decrease after his fifth ride on the Twisty River, we can conclude
a. Sean's marginal utility is starting to increase.
b. Sean's marginal utility has become negative.
c. Sean has maximized his marginal utility