The aggregate demand curve is downward sloping because
a lower price level, holding the nominal quantity of money constant, leads to a larger quantity of money in real terms, causes the interest rate to fall, and stimulates planned investment spending.
a lower price level, holding the nominal quantity of money constant, leads to a larger quantity of money in nominal terms, causes the interest rate to rise, and stimulates planned investment spending.
a higher price level, holding the nominal quantity of money constant, leads to a smaller quantity of money in real terms, causes the interest rate to fall, and stimulates planned investment spending.
a higher price level, holding the nominal quantity of money constant, leads to a larger quantity of money in real terms, causes the interest rate to fall, and stimulates planned investment spending.