A study compared four schools' reading scores. Schools differed in two ways: presence of a librarian (yes/no) and weekly reading-club hours (low/high): School A, librarian + low club hours, 75; B, librarian + high club hours, 82; C, no librarian + low club hours, 70; D, no librarian + high club hours, 73. The researchers concluded that having a librarian has an independent effect on reading scores even after accounting for club hours. The strongest support is ______
Which choice most logically completes the text using the data above?
- A
School B's score of 82.
- Bcheck_circle
the comparison A vs. C (both low club hours: 75 vs. 70) and B vs. D (both high club hours: 82 vs. 73)—librarian adds 5-9 points in both groups.
- C
School C's score of 70.
- D
the gap between A and D.
Explanation
To show librarian has an independent effect, hold club hours constant and vary the librarian variable. Choice B does this in both club-hour strata.