The law of demand is the assertion that a. the quantity demanded of a product is inversely related to its price. b. the demand for a product is negatively related to its price. c. changes in price and changes in quantity demanded move in the same direction. d. the quantity demanded of a product is directly related to its price. an increase in the price of a product causes a decrease in quantity demanded because of the income and substitution effects. more specifically, a. the substitution effect is the decrease in quantity demanded because consumer tastes have changed and the income effect is the decrease in quantity demanded because consumer incomes have fallen. b. the substitution effect is the decrease in quantity demanded because the product is more expensive relative to other goods and the income effect is the decrease in quantity demanded owing to the decline in consumers' purchasing power. c. the substitution effect is the decrease in quantity demanded because there are fewer consumers and the income effect is the decrease in quantity demanded because consumer incomes failed to increase. d. the substitution effect is the decrease in quantity demanded because the consumers' purchasing power is reduced and the income effect is the decrease in quantity demanded owing to the fact that the product is more expensive relative to other goods.