$8.$ Which statement concerning customer valuation and customer equity is true?

a$)$ The cross$-$selling value refers to the monetary success contribution of one customer based on the basic business.

b$)$ The loyalty value refers to customers' "outgoing information".

c$)$ The reference value can be regarded as a direct monetary interaction value.

d$)$ The reference value refers to information within a business relationship $($ingoing information"$).$

e$)$ The cross$-$selling value can be regarded as a direct monetary interaction value.