When the cost of a resource is sunk, then the dual price can be interpreted as the

 a. minimum amount the firm should be willing to pay for one additional unit of the resource. 

b. maximum amount thefirm should be willing to pay for one additional unit of there source. 

c. minimum amount the firm should be willing to pay for multiple additional units of the resource. 

d. maximum amount thefirm should be willing to pay for multiple additional units of there source.