Final answer:
A decrease in price on an upward-sloping demand curve can cause a decrease in quantity demanded due to the substitution effect and the income effect.
Step-by-step explanation:
A decrease in price on an upward-sloping demand curve can cause a decrease in quantity demanded due to the substitution effect and the income effect. The substitution effect occurs when the price of a product decreases, making it relatively cheaper compared to other products, and consumers tend to buy more of the cheaper product and less of the other products. The income effect, on the other hand, means that after the price decrease, consumers have more purchasing power, and they could either purchase the same amount of goods as before and have money left over to buy more or buy more of the product.