The AS curve slopes upward because:

a. per-unit production costs rise as real GDP expands toward and beyond its full-employment level.

b. the income and substitution effects are at work.

c. changes in the determinants of AS alter the amounts of real GDP supplied at each price level.

d. increases in the price level give rise to real-balances effects, interest-rate effects, and foreign purchases effects that increase the amounts of real GDP supplied.