A study found that 70 percent of children surveyed in 1970 had at one time had cavities, whereas only 50 percent of those surveyed in 1985 had ever had cavities. The researchers concluded that the level of dental disease in children had declined between 1970 and 1985.
Which of the following, if true, would most seriously undermine the researchers' conclusion presented above?
(A) Cavities are the most common kind of dental disease to which children are subject.
(B) The children surveyed came from a broad variety of income backgrounds.
(C) The children surveyed were selected from among students of teachers cooperating with the researchers.
(D) The accuracy of cavity detection techniques has improved dramatically since 1970.
(E) The children surveyed in 1985 were younger on average than those surveyed in 1970.