Which statement would most weaken the claim that exposure to varied vocabulary improves reading comprehension?

• A. A large-scale randomized trial shows no effect.
• B. Anecdotal evidence from a single classroom.
• C. A cross-sectional survey across one grade without controls.
• D. Theoretical reasoning lacking data.

Answer: (A)

The important idea is how the design and strength of evidence affect our confidence in a causal claim. A large-scale randomized trial showing no effect directly challenges the idea that exposure to varied vocabulary improves reading comprehension, because randomization and a large sample minimize bias and confounding, so finding no improvement carries substantial evidentiary weight. The other options offer weaker or non-causal forms of evidence: anecdotes from a single classroom aren’t generalizable; a cross-sectional survey without controls can’t establish causation and may reflect preexisting differences; theoretical reasoning without data doesn’t test the claim at all.