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A retailer offered a loyalty discount only to customers who had spent over a threshold in the prior year. After a year, those customers spent more on average than non-members. The retailer credits the discount and plans to extend it to all customers to lift everyone's spending.
Which of the following, if true, would most seriously weaken the argument above?
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Correct answer
C
Membership was defined by past high spending, so members were already top spenders before any discount, which is why they outspent non-members, a built-in selection effect, not proof the discount works. (C) names this directly and explains why extending the discount to everyone need not lift average spending.
(B) concerns a same-direction confound but much smaller than the entry-rule selection that wholly defined the groups. (A), (D), and (E) sit off the selection axis. (C) wins by exposing the threshold-based selection.
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