Which of the Following Is NOT a Reason Why Supply

Which of the following is NOT a reason why supply curves generally slope upward?  

A)  because producers are willing to offer more of a good at higher prices 
B)  because a higher price attracts resources from less-valued uses 
C)  because producers must be compensated for the rising opportunity cost of additional output 
D)  because the price of a good usually must fall to induce an increase in quantity supplied