A study tracked four schools that adopted a new reading curriculum. The schools differed in two ways: curriculum hours per week (3, 5, 5, 8) and class size (28, 28, 18, 18). End-of-year reading gains: School A (3 hrs, 28 students), +2.1 points; School B (5 hrs, 28 students), +3.4 points; School C (5 hrs, 18 students), +5.8 points; School D (8 hrs, 18 students), +7.0 points. The researchers concluded that smaller class size, not just more curriculum hours, drove gains. The data most directly supporting the class-size claim (rather than just hours) is ______
Which choice most logically completes the text using the data above?
- A
School D's largest gain of 7.0 points.
- B
the comparison between A and D, showing both more hours and smaller class size.
- C
School A's smallest gain of 2.1 points.
- Dcheck_circle
the comparison between B and C, where curriculum hours are equal (5) but C's smaller class produced higher gains.
Explanation
To isolate class size, one must hold curriculum hours constant. Schools B and C both had 5 hours but different class sizes (28 vs. 18), with C scoring higher. Choice B uniquely isolates class size as the driver. Choice A confounds the two variables.