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

Multiple Choice

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

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