Figure 8-5 Panel (A) Shows an Economy's Aggregate Production

Figure 8-5 Panel (a) shows an economy's aggregate production function, Panel (b) shows the labor market and Panel (c) shows the economy's long-run aggregate supply curve.
-Refer to Figure 8-5. In Panel (c) , the long-run aggregate supply curve is vertical because

A) of diminishing marginal returns.
B) of sticky wages and prices.
C) perfectly flexible nominal wages in the long run allow any number of wage/price level combinations to result in a real wage that equilibrates the labor market.
D) the economy is not constrained by diminishing marginal returns since it can change the quantities of all factors of production.