Assume that Vincent's customers are always available for the tour; therefore, he can fill his bus for each tour as long as there is sufficient total demand for the day. Vincent is considering changing his pricing strategy. Which of the following options results in the highest profit per day? a. Charge a single price of $10 to all passengers. b. Charge a single price of $12 to all passengers. c. Charge a single price of $18 to all passengers. d. Continue charging each buyer his/her willingness to pay