$24.$ Which statements concerning customer valuation and customer equity is true?

a$)$ The cross$-$selling value concerns future, successive, additional purchases, which have not been received yet.

b$)$ The reference value refers to customers' "ingoing information".

c$)$ The basic value describes the direct monetary success contribution of all customers based on the basic business.

d$)$ Customer equity is the contribution made by one customer to increase the firm's value.

e$)$ The information and the cooperation value can be regarded as direct monetary interaction values.