It is not rare in that people in fifties are going to university for further education.

问题补充：

It is not rare in that people in fifties are going to university for further education.

A. 90s; the

B. the 90s; 不填

C. 90s; their

D. the 90s; their

答案：

A. 90s; the

B. the 90s; 不填

C. 90s; their

(答案→)D. the 90s; their

解析：表示在几十年代，在以 ty 结尾的复数基数词前加 the; 表示在某人几十多岁时，用 one’s 加以 ty 结尾的复数基数词。