The Aggregate Supply Curve Cannot Be the Sum of the Supply

The aggregate supply curve cannot be the sum of the supply curves of all the individual firms in the economy because

A) some firms actually set prices instead of just responding to process determined in the market.
B) the outputs of some firms are the inputs of other firms.
C) it is unrealistic to believe that costs are constant for individual firms if the overall price level is increasing.
D) all of the above