Free GMAT Critical Reasoning Practice Question

The conclusion is causal: the discount reduced churn. The chief threat to that claim is that the two groups may have differed before the offer, so the lower churn in the discounted group could just reflect who was in it rather than anything the discount did. This is the selection or confounding problem: when assignment is not random, a baseline difference can masquerade as an effect.

(C) closes exactly this gap. Drawing both groups at random from subscribers with similar tenure and viewing habits means the groups were comparable to begin with, so the discount becomes the most plausible source of the churn difference. That is the direct strengthener.

(A) concerns how the discount was funded, which has no bearing on why subscribers stayed. (B) reports market size, which is impressive context but says nothing about whether the groups were alike. (D) notes the discounted group was told the offer was a limited promotion; rather than help, this introduces a competing cause (urgency from the framing) and still does not address baseline comparability. (E) is the strongest distractor: consistent measurement is genuinely necessary for the comparison to be fair, so it is tempting to treat it as decisive. But it only rules out a measurement artifact; it leaves open that the groups differed before the offer, which is the central worry. A necessary condition is not a sufficient one, so (E) does not establish causation. Only (C) eliminates the baseline-difference alternative, so (C) is correct.