What Is the Theory Behind the Incremental Productivity Method

What is the theory behind the incremental productivity method?

A) tallying the average number of sales calls needed to close each individual sale
B) estimating the effect of additional sales on the motivation of the salesperson
C) comparing the additional cost of one new salesperson to expected additional sales revenue
D) estimating the number of products sold in relation to the size of the sales territory
E) matching the sales force size to a given geographic area