Which of the Following Is an Example of Marginal Analysis

Which of the following is an example of marginal analysis?

A) To determine the optimal number of workers, a firm calculates the net benefits of hiring an extra worker.
B) To determine the optimal amount of fertilizer to be used, a farmer calculates the total benefits of using a given amount of fertilizer.
C) To choose the optimal apartment to rent, an individual estimates the net benefits of renting an apartment close to his place of work.
D) To determine the optimal output that a firm should sell, the manager calculates the total revenue earned by selling different levels of output.