PrepTest 122, Section 1, Question 2
A study of plaque buildup on teeth used(83 more words not shown)
Which one of the following,(17 more words not shown)
- AThe buildup of(28 more words not shown)
- BThe stiffness of(20 more words not shown)
- CThe people who(18 more words not shown)
- DBefore they joined(22 more words not shown)
- EThe people in(17 more words not shown)
Reading the argument
A new toothbrush might help. But why would it help more than cleaning the old one?
At first, I might expect three different results: the unchanged toothbrush does worst, the sterilized toothbrush does better, and the new toothbrush does best. That's a guess about what cleaning and replacing the brush might accomplish. The results don't follow that pattern. Sterilizing the old toothbrush makes no apparent difference; replacing it does.
Think about an old toothbrush with its bristles bent and spread out. Sterilizing it wouldn't straighten those bristles. A replacement gives us a fresh brush as well as a clean one. Maybe the brush wears out, and that's why replacing it works better than simply sterilizing it.
What we’re looking for
We're asked to explain the results for all three groups. I want something that separates Group 3 from both other groups. A fact about cleanliness alone may separate Group 1 from Group 3, but it still needs to account for Group 2, whose brush was sterilized every month.
Working the answers
- A
This could explain why Group 1 does worse than Group 3: Group 1 lets bacteria build up on an unsterilized brush. But Group 2 sterilizes its toothbrush every month and still has as much plaque as Group 1. If unsterilized brushes were the explanation, why doesn't Group 2 share Group 3's advantage? A explains only part of the comparison.
- BCorrect
Stiff bristles help prevent plaque, and repeated brushing wears away that stiffness after several months. Groups 1 and 2 keep using the same aging brush all year. Cleaning Group 2's brush doesn't restore its stiffness. Group 3 replaces the brush monthly, before those several months of wear accumulate, so its members keep the useful stiffness. That explains why replacement helps when sterilization alone doesn't.
- C
Using a different measuring method doesn't explain a difference among groups measured in the same study. We'd need to know something about how that method affects the comparison. Merely saying the method is new doesn't tell us why Group 3 has less plaque.
- D
These people's earlier habits don't explain why Group 3 ends the study with less plaque than the other groups. During the study, everyone brushes twice daily. D doesn't give us the groups' starting plaque levels or a reason the change from once to twice daily would produce the reported difference.
- E
Equal effort removes one possible explanation: Group 3 wasn't simply brushing harder. But ruling out that possibility doesn't tell us what did make Group 3's results better. We need a reason the new brushes help, and B supplies it.
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