Production and Cost - Economic cost can best be defined..?

Since my test is close and I need to get ready for different subjects. It would be useful for me on the off chance that you could give me an answer to the theme. 

Economic cost can best be defined as: 
A. any contractual obligation that results in a flow of money expenditures from an enterprise to resource suppliers.
B. any contractual obligation to labor or material suppliers.
C. payments that must be received by resource owners to insure the resources' continued supply.
D. all costs exclusive of payments to fixed factors of production.

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