If Firms Set Prices and Then Keep Them Fixed for a Period

If firms set prices and then keep them fixed for a period of time, their fixed prices imply that

A) the aggregate price level is fixed and that aggregate demand determines the quantity of goods and services sold.
B) the aggregate price level adjusts continuously.
C) prices are set by aggregate demand and supply.
D) the aggregate price level is fixed and that aggregate supply determines the quantity of goods and services sold.