By looking at aggregate demand via its component parts, we can conclude that the aggregate demand curve is downward sloping because
a. a lower inflation rate causes the real interest rate to fall, and stimulates planned investment spending.
b. a lower inflation rate causes the real interest rate to rise, and stimulates planned investment spending.
c. a higher inflation rate causes the real interest rate to fall, and stimulates planned investment spending.
d. a higher inflation rate causes the real interest rate to rise, and stimulates planned investment spending.

A

Explanation:

The aggregate demand downward slopes due to a number of reasons .The downward sloping aggregate demand curve means that as the price level drops, the quantity of output demanded increases. Similarly, as the price level drops, the national income increases.

The first reason for the downward slope of the aggregate demand curve is Keynes's interest-rate effect. Recall that the quantity of money demanded is dependent upon the price level. That is, a high price level means that it takes a relatively large amount of currency to make purchases. Thus, consumers demand large quantities of currency when the price level is high. When the price level is low, consumers demand a relatively small amount of currency because it takes a relatively small amount of currency to make purchases. Thus, consumers keep larger amounts of currency in the bank. As the amount of currency in banks increases, the supply of loans increases. As the supply of loans increases, the cost of loans--that is, the interest rate decreases .

Another reason for the downward slope of the aggregate demand curve is Mundell-Fleming's exchange-rate effect. Recall that as the price level falls the interest rate also tends to fall. When the domestic interest rate is low relative to interest rates available in foreign countries, domestic investors tend to invest in foreign countries where return on investments is higher. As domestic currency flows to foreign countries, the real exchange rate decreases because the international supply of dollars increases. A decrease in the real exchange rate has the effect of increasing net exports because domestic goods and services are relatively cheaper. Finally, an increase in net exports increases aggregate demand, as net exports is a component of aggregate demand. Thus, as the price level drops, interest rates fall, domestic investment in foreign countries increases, the real exchange rate depreciates, net exports increases, and aggregate demand increases.

Lastly,the reason for the downward slope of the aggregate demand curve is Pigou's wealth effect. Recall that the nominal value of money is fixed, but the real value is dependent upon the price level. This is because for a given amount of money, a lower price level provides more purchasing power per unit of currency. When the price level falls, consumers are wealthier, a condition which induces more consumer spending. Thus, a drop in the price level induces consumers to spend more, thereby increasing the aggregate demand.