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To validly attribute the performance difference to the study method (spaced repetition vs. cramming), potential confounds must be controlled. Choice A establishes group equivalence at baseline, ruling out preexisting differences as the explanation for post-test outcomes. With comparable initial knowledge, the superior recall can more plausibly be credited to spacing. This aligns with LSAT objectives on experimental reasoning: strengthening causal claims by ensuring randomization or equivalence of groups and eliminating alternative explanations.

Choice B introduces an ancillary feature (gamification) that could confound the causal inference by making the app more engaging; it does not strengthen the inference without showing it is irrelevant. Choice C references anxiety but doesn’t connect it causally or show group equivalence in other respects. Choice D is background information about the importance of vocabulary, not about causation in the study. Thus A best supports the causal conclusion by reinforcing internal validity—the core mechanism tested in questions about studies and experiments.