Which of the following formulas best represents the customer lifetime value $($CLV$)?$

Question $26$ options:

a$)$

CLV $=$ npv $($sales $$ costs$)$ npv $($equity$)$

b$)$

CLV $=$ npv $($sales $$ costs$)/$ npv $($fixed cost$)$

c$)$

CLV $=$ npv $($sales $$ costs$)+$ npv $($equity$)$

d$)$

CLV $=$ npv $($sales $$ costs$)$ npv $($fixed cost$)$