In the Equation to Calculate the Economic Value of a Customer

In the equation to calculate the economic value of a customer, V = R × F × M / D, how is the value for F estimated?

A) It is estimated to be the total number of purchases the customer has made.
B) It is estimated to be the number of visits of the customer without actually spending on an item.
C) It is estimated to be the purchase frequency per year.
D) It is estimated to be the number of customers defecting per year.