A cost that has already been incurred and cannot be changed is called a$($n$)$:

A$.$ Opportunity cost.

B$.$ Out$-$of$-$pocket cost.

C$.$ Joint cost.

D$.$ Sunk cost.

Costs that have not yet been incurred and that may vary among different courses of action

are called:

A$.$ Opportunity costs.

B$.$ Out$-$of$-$pocket costs.

C$.$ Joint costs.

D$.$ Sunk costs.

Incremental revenues:

A$.$ Always increase revenue when one course of action is selected over another.

B$.$ Always decrease revenue when one course of action is selected over another.

C$.$ May increase or decrease when one course of action is selected over another.

D$.$ Cause revenues to remain steady.