A study of plaque buildup on teeth used three randomly assigned groups of people who brushed their teeth twice a day for a year. ██████ ██ █████ █ ████ ███ ████ ██████████ ███ █████ ██████ ██ █████ █ ████ ███ ████ ██████████ ███ ████ ███ ██████████ ██ ████ ██████ ██████ ██ █████ █ ████ █ ████ ███████ ██████████ ████ ██████ ██ ███ ███ ██ ███ █████ ██████ ██ ██████ █ ███ █ ███ ███ ████ ██████ ██ ██████ ███████ ██ ████ ██████ █████ ██████ ██ █████ █ ███ ████ ██████ ████████
Why did people in Groups 1 and 2 have the same amount of plaque buildup after a year while people in Group 3 had less?
The correct answer choice must provide a similarity between people in Group 1 and Group 2 that helps explain why they had the same level of plaque buildup, a difference between people in Group 3 and those in Groups 1 and 2 that helps explain why people in Group 3 had less plaque buildup, or both.
Which one of the following, ██ █████ ████ █████ ██ ███████ ███ ████████ ███████ ██ ██████ ███████ █████ ██ ███ █████ ███████
The buildup of ██████ ██ ██████ █████ ████████ █████ █ ███ █████ ██ ████████ ██ ███████████ ██ ███ ██████ ██ ████████ ██ ████████████ ████ ████████ ████████████ ███ ████ ████ █ ██████
People in Group 2 sterilized their toothbrushes monthly but had the same level of plaque buildup as those in Group 1. Also, people in Groups 2 and 3 used sterilized toothbrushes to begin each month, but they had differing buildup levels. (A) explains none of these things.
The stiffness of ███ ████████ ██ ███ █████████████ █████ ███ ██████████ ██████ ██ ████████ ████████ █████ ███████ ███████ ████████ ███ ███████ ██ ███████
This gives us a key difference between the experience of people in Group 3 from the experience of people in Groups 1 and 2. Only those in Group 3 used a new toothbrush each month, so (B) helps explain why people in Group 3 had less plaque buildup than those in Groups 1 and 2.
The people who ███ ███ █████ ████████ ███ ██████ ██ ██████ ███████ ██ █ ███ ██████ ███ ███████ ████████ ██ █████████
This new method was used to measure the plaque buildup of all people in each Group, so it doesn’t help explain our phenomena.
Before they joined ███ ██████ ████ ██ ███ ██████ ██ █████ █ ███ ████ ██ ███ █████ ██ ████████ █████ █████ ████ ████ █ ████
What people were doing before the study is irrelevant. We want to know why people in Groups 1 and 2 saw the same level of plaque buildup during the study, people in Group 3 saw less buildup than those in Groups 1 or 2, or both.
The people in █████ █ ███ █████ █ ███████ █████ █████ ██ ██████████ ██ ███ ███ ██████ ██ █████ ██
(E) gives us a similarity between all 3 groups. It explains nothing about why Groups 1 and 2 saw the same amount of plaque buildup even though they took different approaches to teeth cleaning nor why people in Group 3 saw different levels of buildup than those in Groups 1 and 2.
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