$26.$ Which of the following statements concerning the customer lifetime value $($CLV$)$ is false?

a$)$ The CLV of a customer can be positive, negative and zero.

b$)$ Due to their lower complexity, practitioners use brand$-$switching models more often than other CLV models:

c$)$ Customer retention models typically account for mid$-$period advertisement casts while revenues are accounted for at the beginning of the period.

d$)$ The "lost$-$for$-$good" assumption leads to models of customer behavior involving a constant customer retention rate.

e$)$ The CLV is a dynamic method to determine the monetary value of a customer.