Which of the following best describes why the aggregate demand curve is downward sloping?
a. as the aggregate price level decreases, the stock of existing physical capital increases.
b. as the aggregate price level increases, consumer expectations about the future change.
c. an increase in the aggregate price level causes consumer and investment spending to fall because consumer purchasing power decreases and money demand increases.
d. as a good\'s price increases, holding all else constant, the good\'s quantity demanded decreases.