How Would a Marketing Manager Determine a Firm's Retention Rate

How would a marketing manager determine a firm's retention rate?

A) by determining the percentage of existing customers that a firm keeps as compared to its total number of customers
B) by figuring out the expense to the firm of attaining a new customer
C) by considering the revenues and costs of retaining customers over the long term
D) by analyzing the firm's customer base as compared to the forecast in the firm's marketing plan
E) by determining the average profit margin for each of the firm's customers per year