Explain why the following statements are false.

a. “The aggregate-demand curve slopes downward because it is the horizontal sum of the demand curves for individual goods.”

b. “The long-run aggregate-supply curve is vertical because economic forces do not affect long-run aggregate supply.”

c. “If firms adjusted their prices every day, then the short-run aggregate-supply curve would be horizontal.”

d. “Whenever the economy enters a recession, its long-run aggregate-supply curve shifts to the left.”