Question ID:PT41 S1 Q2 Passage: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. Stem:Which one of the following, if true, most helps to explain the relative amounts of plaque buildup found in the three groups? Correct Answer Choice:BChoice A:The buildup of plaque on teeth, which brushing twice a day helps to prevent, is accelerated by the growth of bacteria on toothbrushes that remained unsterilized for more than a month. Choice B:The stiffness of the bristles on new toothbrushes, which the mechanical action of brushing destroys after several months, inhibits the buildup of plaque. Choice C:The people who did the study measured the amount of plaque buildup by a new method not usually employed by dentists. Choice D:Before they joined the study, some of the people in Group 3 had been in the habit of brushing their teeth only once a day. Choice E:The people in Group 2 and Group 3 brushed their teeth as vigorously as did the people in Group 1.