AD shortfall, economics homework help

In a diagram of aggregate demand and supply curves, the AD shortfall is measured as the: (Points: 1)

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.