Which bests defines the term "sunk costs"?

Select one:

a. Non-cash costs that are important only because they serve as a tax shield

b. Costs that have already been incurred and should not change behavior

c. Costs that are so large that they tend to overwhelm the project

d. Benefits that would not have been received or costs that would not have incurred if a project had not been undertaken

e. Hidden but relevant costs or benefits that should be factored into cash flows