单选题 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、 (E) The children surveyed in 1985 were younger on average than those surveyed in 1970.

【正确答案】 E

【答案解析】[解析] 本题属于“B，A”型，反对的思路是“有他因”。(E)指出1985年调查有牙洞的孩子减少是因为1985年调查的孩子要比1970年调查的孩子要小，因此使得段落推理结论“孩子中的牙病水平降低了”被削弱；(A)、(B)与“牙病水平的降低”无关；(C)没有区分1970年与1985年；(D)起到了支持作用。