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.

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.

This new method was used to measure the plaque buildup of all people in each Group, so it doesn’t help explain our phenomena.

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.

(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.