Every Time Beth Buys a Book at Waldenbooks,she Presents Her

Every time Beth buys a book at Waldenbooks,she presents her Waldenbooks card,and the sales associate enters her purchase in a database.When she has purchased five books at regular price,she is sent a coupon for a free book to be picked out on her next visit to the store.This is an example of a:

A) bonus program.
B) frequency program.
C) customer rewards contest.
D) self-liquidating promotion.
E) subsidized program.