(D) is incorrect because it has no impact on the argument. The study measured the plaque buildup over the course of a year using the beginning of the study as the benchmark. The habits of any study participants prior to the study are irrelevant. Moreover, (D) only talks about :"some" people in group 3, which could be as few as one person, and the habits of "some" people cannot explain the outcome of the study as a whole.
