2. A study of plaque buildup on teeth used three randomly
assigned groups of people who brushed their teeth
twice a day for a year. People in Group 1 used the same
toothbrush all year. People in Group 2 used the same
toothbrush all year but sterilized it each month. People
in Group 3 used a new, sterile toothbrush each month.
At the end of the year, people in Groups 1 and 2 had
the same amount of plaque buildup as each other, while
people in Group 3 had less plaque buildup.
Which one of the following, if true, most helps to explain
the relative amounts of plaque buildup found in the three groups