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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 in the loyalty program was defined by past spending above a threshold, so members were already the retailer's top spenders before the discount could have any effect. That built-in selection effect, not the discount, is why members outspent non-members. (C) names this directly and explains why extending the discount to everyone need not lift average spending.
(B) points to a real confound, marketing emails sent only to members, but it is a smaller, secondary factor layered on top of a baseline gap that the entry rule alone already explains in full. (A), (D), and (E) sit off the selection axis entirely. (C) wins by exposing the threshold-based selection that defined the two groups from the start.
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