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To undermine a causal claim, present a credible alternative explanation tied closely to the measured outcome. Option A shows that the increase is limited to customers targeted by a concurrent coupon mailer, a separate intervention that plausibly explains repeat purchases. If true, this co-movement compromises the attribution to the app. Option B (satisfaction) is orthogonal to causality; users might like the app without it driving behavior. Option C is background context about competitors and timing; it doesn’t address whether our increase was caused by our app. Option D (overall traffic up 2%) is weak and nonspecific; the outcome at issue is repeat purchases, not footfall. On GRE Argument weaken questions, the strongest rebuttals identify confounds temporally aligned with the effect and specific to the population in which the effect appears, thereby severing the cause–effect link.

Formula for sum of nn terms: Sn=n2(2a+(n−1)d)S_n = \frac{n}{2}(2a + (n-1)d). Given S20=710S_{20}=710, a=5a=5, n=20n=20.

710=202(2⋅5+19d)=10(10+19d).710 = \frac{20}{2}(2\cdot 5 + 19d) = 10(10 + 19d).

Divide both sides: 71=10+19d⇒19d=61⇒d=61/19=3.2171 = 10 + 19d \Rightarrow 19d = 61 \Rightarrow d = 61/19 = 3.21. Wait, check arithmetic: 710/10=71710/10 = 71. Yes. Then 71=10+19d⇒61=19d⇒d=61/19=3.2171 = 10 + 19d\Rightarrow 61=19d\Rightarrow d=61/19=3.21. That is not an integer. Re-check setup. Formula correct. Let’s try alternative: Average of first and last term × number of terms. Let last term = a+19da+19d. Then sum = (20/2)(first+last) = 10(5 + (5+19d)) = 10(10+19d). Same as before. So correct. Compute again: 710/10 = 71. 71–10=61. 61/19=3.21, not neat. But 61/19=3.2105, maybe answer choices? They gave 2,3,4,5. Closest is 3. Did I mis-copy? Let’s recompute carefully. If d=3, last term=5+193=62. Sum = 20(5+62)/2=2067/2=670. Too small. If d=4, last=81. Sum=20(86)/2=860. Too large. So actual is between 3 and 4. Correct d=61/19=3.21. Maybe test intends 3.2 ~ 3.21 → not listed properly. The mathematically correct common difference is 61/19 ≈ 3.21.