For each of the following statements, indicate whether it is true or 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 the price level does 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.