The correct answer is C.
When economic decisions are analyzed, this is typically done "at the margin". Therefore, the consequences of adding or substrating an aditional unit of a certain variable are analzyed and, depending on whether the total benefits have been increased or decreased by such adition/substraction, that action will be undertaken in practice or not.
For example, the marginal cost of producing an extra unit is calculated and, if it does not exceed the revenue earned by selling that extra unit, then the firm will decide to produce that aditional item. If the marginal cost exceeded the marginal revenue, such item would not be produced.
