A relevant cost is best defined as a ____________ that _________between alternative decisions, and ___________ is not normally considered as a relevant cost for decision-making.

a. past cost ; changes ; an incremental cost

b. past cost ; changes ; a sunk cost

c. future cost ; changes ; a sunk cost

d. future cost ; stays the same ; an opportunity cost

e. future cost ; changes ; an incremental cost

f. past cost ; stays the same ; an opportunity cost