13 Which of the following describes a situation in which demand must be inelastic? a. Total revenue decreases when the price of vitamins rises. b. Total revenue does not change when the price of vitamins rises. c. The price of vitamins rises by 10 cents, and quantity of vitamins demanded falls by 50. d. The price of vitamins rises by 10 cents, and total revenue rises. e. A 20 percent increase in the price of vitamins leads to a 20 percent decrease in the quantity of vitamins demanded