In a Diagram of Aggregate Demand and Supply Curves,the GDP

In a diagram of aggregate demand and supply curves,the GDP gap is measured as the

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