Challenge Your Exam Readiness With Our Free NDRT Practice Questions

The passage contrasts observational associations with randomized trials. It notes that supplement users differ systematically (exercise, income), making it unclear whether the supplement or lifestyle drives longevity. Randomization assigns treatments by chance, balancing such covariates, so any remaining difference is more plausibly due to the supplement. Therefore, B captures the methodological point: randomization distinguishes causal effects from confounding. Option A overreaches; observational studies can generate hypotheses. Option C introduces a normative claim not discussed. Option D is unfounded; the passage neither critiques nor defends placebo ethics. For Nelson–Denny main-idea items, look for the sentence that articulates the procedure’s purpose (“equalize such traits on average”) and the conditional interpretation that follows. Recognizing research-design vocabulary—confounding, randomization, placebo—helps you track why certain methods earn stronger causal inferences, a frequent theme in science-and-society passages.