A student was asked to draw a demand and supply graph to illustrate the effect on the personal computer market of a fall in the price of computer hard drives, ceteris paribus. He explained it as follows: Hard drives are an input to PCs, so a fall in the price of hard drives will cause the supply curve for personal computers to shift to the right. Because this shift in the supply curve results in a lower price, consumers will want to buy more PCs and the demand curve will shift to the right. We know that more PCs will be sold, but we can't be sure whether the price of PCs will rise or fall. That depends on whether the supply curve or the demand curve has shifted further to the right. I assume that the effect on supply is greater than the effect on demand, so the final equilibrium price will be lower than the initial equilibrium price. "