an increase in the price of gasoline shifts the demand for tires in which direction? a to the right, because an increase in the price of gasoline makes consumers poorer and thus not willing to pay as much for tires. b to the left, because gasoline and tires are substitutes. c to the right, because gasoline and tires are normally used together. d to the left, because gasoline and tires are normally used together. e to the right, because gasoline and tires are substitutes.