Why Does the Aggregate Demand Curve Slope Downward

Multiple Choice

Why does the aggregate demand curve slope downward?  

A)  because when the price level increases, consumers feel poorer and therefore buy less 
B)  because when the price level increases, consumers feel richer and therefore buy more 
C)  because when domestic prices increase, consumers substitute domestic goods for imported ones 
D)  because when the price of a particular good increases, consumers substitute away from that good