In a diagram of aggregate demand and supply curves, the AD shortfall is measured as the:

A) Vertical distance between the equilibrium price and the price at which the aggregate demand would intersect aggregate supply at full employment.
B) Horizontal distance between the equilibrium output and the full-employment output.
C) Horizontal distance between the aggregate demand curve necessary for full employment and the aggregate demand curve at the equilibrium price.
D) All of the above.