Answer choice (B) is incorrect because it does not weaken the argument. The author of the passage concludes that Activite must be effective because the company would not otherwise offer a free month's supply to new customers.
The fact that there may be less expensive dietary supplements that are just as effective as Activite does not impact the author's conclusion that Activite must be effective. Nowhere in the passage do the author or the company claim that Activite is the cheapest or most effective brand on the market.
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