The Economy's Aggregate Supply (AS)curve Is Assumed to Slope Upward

The economy's aggregate supply (AS) curve is assumed to slope upward because

A) inputs become less expensive at higher levels of output.
B) firms' unit costs fall as output increases.
C) inputs become more expensive at higher levels of output.
D) aggregate demand increases at higher levels of national income.
E) firms' unit costs rise as output increases.