参考答案和解析
正确答案：
A
解析：
语义题。“people in this age group”出现在文中第二段第三句，该句指出“One exception is found among people in their twenties…, because people in this age group are most likely to have young children.”，由于“people in this age group”出现在该句中的原因状语从句中，可知该词组指的是前面提到的people in their twenties，故选B。
